Where do you get Blueprints?
You loot them by crafting stuff yourself, or you buy them off of others. Auction has a lot of blueprints always for sale and many shops. You can also look around on forum auctions to find some too.
Oh, and keep an eye out on the ground. Some crafters hate selling blueprints to the TT and hate the hassle of selling them on the auction house, so they just throw them on the ground for others to pick up. Does not happen too often, but does happen a lot more often then you'd think it does.