Speculah:C.I.Akemi - Homura's intel source
Homura Akemi has, ever since the start, displayed more knowledge than any other character, with the exception of Kyuubey. At the same time, speculation grew that she was a time-traveler. That was confirmed in Episode 10, along with a depiction of her past experiences in the repeating timeline, with the accumulated knowledge from those repetitions explaining how she is able to predict almost every occurrence in the current timeline. Thus, controversy on the origin of all that information subsided. Well, mostly.
However, the time reset does not explain everything.. There are several pieces of intel that Homura couldn't possibly have obtained through the recurring events in Mitakihara in that fifteen day period. From her experience, she only knew that Kyuubey had tricked the girls, and the consequences and mechanics of the cycle (but not the reasoning behind it; and we have no information whether she knows it's purpose if to combat entropy). She must have gotten that information elsewhere.
Examples
- Kyuubey's real name
- Walpurgis' past occurrences in other cities
- The files and pictures of other witches
Analysis
Let's analyze those three.
The first is the most significant one. It's utterly impossible that she'd obtain that information from anyone on the current cast. The only one who has learned it so far was Madoka, during Kyuubey's explanation of the magical cycle, where he identified his race as "Incubators". However, it is safe to assume that it is the first time he does it, given the specific circumstances of this timeline. And that was only after Homura revealed she knew that. The only other character that could have told her is Kyuubey. But there is simply no logical reason for that. Given that she had already contracted, he had absolutely no need to tell her that. One might argue he might have told her at a time in that timeline where it did no longer mattered, for example, after Madoka turned into a witch. It is something we usually see both in movies and real life. However, it requires at least one of two things: pity for the defeated adversary, and pride for knowing you've vanquished your adversary, and can speak the truth knowing they cannot do anything about it. Kyuubey can feel neither one nor the other. He he cold and emotionless: even if there was the slightest chance Homura could do something with that information, he would not tell her, ever. Another argument is that she could have continuously asked if this or that were true, and he'd tell her when she guessed it. I don't think so, as I do believe Kyuubey is very well capable of lying. This is more of a personal opinion, though.
The second and third are not as strong, but still valid. Files and pictures, so-called "statistics" by Homura are definitely easier to obtain that an unknown alien's real name. However, the amount of such files is massive. She possesses a wall chock full of it, plus those on the table. And this is not counting possible absent files, and other information she obtained, without the actual file (that is, things she heard or saw herself). And all of this in the same repeated fifteen days. So, where did she obtained those files? The obvious answer would be other magical girls, and possibly human reports of paranormal activity that matches the description of a witch attack (police reports, files from occultism clubs, etc.). Kyoko, being her partner in this timeline, simply does not seem to be the kind to write and keep reports. Neither did Mami, the only other girl in the cast that could have this information (assuming Homura would loot her apartment for them). Kyuubey, of course, has all the information, but would not give a dime of it, ever.
Who could it be?
Getting to the main point: if this mysterious source does exist, who could it be?
I see only two possibilities: a foreign, veteran magical girl, or another Incubator. Or an archive/intel cache left by either of them.
The first one is rather simple: a magical girl from another city (or maybe not, simply laying low on the streets of Mitakihara), who learned the truth somehow and lived through it, archived and provided her with information. Possibly on her deathbed, wanting to avenge her, or maybe moved by Homura's story (assuming she'd tell her). It leaves the question how did they meet up. Either Homura heard rumors of her, or she was the one that sought after Homura.
The second is more intricate. A second Incubator provided her with all the information on her species and Kyuubey's plans. Why would he betray a member of their own species? There are some possibilities. Perhaps the Incubators are peaceful, even if emotionless, and actively investigating a way to stop, delay, or reverse entropy. They discovering the magical system, but discarded it due to to it's consequences, and proceeded to find other methods. Kyuubey, being the asshole he his, refused to accept that, and started making use of the magical system as means to obtain energy. This could have been done either under the Incubator's noses, or at open sight, making him a rogue element of his race. Thus, an Incubator was sent to Earth (or, in the undercover possibility, an Incubator that discovered the truth alone) to stop Kyuubey's madness. Having made contact with Homura in one of the timelines, he transmitted all useful information to her.
An alternative could be the opposite: the race itself does not care about the means to obtain energy, but one member developed emotions (in a shout out to V), and went on a quest to stop the use of this abhorrent method. In this case, the "good" Incubator would be the rogue.
Finally, it could simply be an Incubator with a grudge/personal rivalry with Kyuubey.
Whatever the case, it's needless to say the information obtained from another Incubator is, of course, invaluable.
All of this is, obviously, assuming the Incubators are separate individuals and not some sort of hive mind.
The main problem to this theory is that, poorly executed, would lead to a horrible ending. But if there is one great thing about Madoka, is that the story is good, but the execution is flawless. So, I wouldn't be worried about that.
Just Because It's Not On-Screen...
Doesn't mean it didn't happen.
In all past timelines we have only gotten to see key events (literally 3-5 minutes of each at most). Timeline 1 obviously doesn't work here, and it's at the very end of Timeline 2 where she found out what Kyubey was up to. How do we know for a fact she didn't just ASK Kyubey during Timeline 3 and 4 for the full details? We have seen for a fact that Kyubey doesn't with-hold information (much) when asked, so really, all she'd have had to do is "hey, Kyubey, why are you doing all this".
Furthermore, some of Homura's lines contradict the possibility of her getting her intel from other sources: "I won't rely on anyone else anymore." It seems more likely that everything she has she has gotten through the timeloops and, well, THE INTERNET (in the case of: how to make bombs, Walpurgis Night's significance, paintings, etc).
Frankly it just seems more likely that she got all of that info herself. The info on Walpurgis Night is easy enough to get considering she's lived to see it happen four times already, and the girl can stop time, so there's no information she couldn't get her hands on if she were willing to use her magic to get it. Her imagery on which girl became which witch could simply be research, or again, maybe she just asked Kyubey "hey, which girl was this before" in one timeline and then researched it in another.
TL;DR: I'm not seeing anything here that dictates that Homura couldn't have gotten any of this information herself. That she did should be a sound theory and not something that's impossible.
Alternative Questions
That are much easier to question than her ability to gain knowledge through timelooping:
"How did she make her apartment look like that?"
"What the Hell is she using to make her pipe bombs if it's an internet recipe and the blast radius is large enough to engulf entire witches?
"What's up with the ridiculously complex pendulum in her apartment and did she build it herself?"
"How is she making that intel she found float in the air like that?"