I buy the thistle in a 25 pound bag from the feed store. The brand is Royal Wing. It's a Tractor Supply store, but it's called Del's Farm and Feed. They carry the Tractor Supply stuff though. Not sure if the seed would be more regional or not. It's pretty much the cheapest place I've found it, but it is still expensive. It's usually about $30 a bag, but I always wait until it goes on sale and stock up on a few then. I did find it at another feed store yesterday, and it is just a tiny bit cheaper, at $50 for 40 pounds, but it is only a tiny bit cheaper that way, and I can't remember the brand on that one. I like the 25 pound bags though. They are easier to deal with. And, if I can get them on sale for around $20, it isn't too bad. It's expensive, but not enough to make me stop buying it.
The other thing I buy is a 50 pound bag of white proso millet, for $21, from the other feed store. Yes, I can probably buy cheaper mixed food, but so much of the cheap food just sits there without being eaten. So, I think it is a much better deal to buy what the birds like. I mix it with black oil sunflower seeds, and every last bit is eaten.