The source is bot farms located in countries with favorable extradition laws (or none at all). That means if an American social media corp, like Facebook, wants to sue them, they can't. Or rather they can, but they would waste their time & resources and achieve nothing. These countries are usually in Asia, Eastern Europe or South America. These bot farms supply over 80% of the retail SMM panels. The rest, the ~20%, is in-house, meaning the panels have their own bot farms. You can easily identify panels with in-house bots, because their prices will always be much higher.
Now, you may be asking: how to find the bot farms and skip the middleman? The answer is simple - you don't find them, they will find you. Here's what you do:
1) Pretend you are a SMM reseller. Build a website, make it look nice, include the services you want, put average prices from comps, etc.. Make hundreds of fake orders with your own money (you will only pay CC commission rates, so on $1k you lose about $30-40). You don't have to open a business - a website and financials are enough.
2) Get some real traffic, so the "big guys" (those who own bot farms or 1-2 levels below them) notice you and contact you with their offerings. A couple of options:
a) Post in every marketing forum, starting with the big ones. Make a thread saying you are looking for a supplier, that you already have a business receiving hundreds of orders per day and are looking to scale up & improve your margins. Make a video showing your financials. Wait for the PM-s to come in. The ones with the cheapest prices you probably can't find anywhere else are the ones you are looking for. Enjoy!
b) If option A doesn't pan out, rank your site in Google for SMM-related keywords. Have a professional contact page and wait for emails to come in. The more profitable keywords you rank, the more the chances you will get contacted. However, this option is only for a minority, as you have to actually build the SMM business, at least long enough until you get your info. This means completing real orders that come through and ranking in Google, which is another challenge of its own.
Now, something to be aware of: finding the bot farms does not necessarily mean you are going to do business with them. These guys deal with quantity, hence why you can get the lowest prices possible. And when I say quantity, I mean at the very least tens of thousands of orders, not a couple thousand likes at your pages. This is the reason why you put fake orders in the financials above. So even if you find them, unless you actually run a SMM business, you will NEVER do business with them.
Think of it like the illegal drugs business. There is a drug lord who supplies everyone else, with a lot of layers in between the lord and the simple retail guys in the end who sell it on the streets to the consumer. If you happened to find the drug lord, do you really think you could buy just a few bags of weed from him? Nope, you would just get shot in the face, in this case literally