Greetings, fellow Kansan! Welcome!
It probably depends on what exactly you're looking for. If you just want the most basic of material starting from ground zero, there are a ton of plant websites you could use. I've had very good luck with eBay in the past for purchasing many plants including Bougies, and there is a ton of very simple material with blooms of all colors. They generally just amount to small rooted cuttings up to some larger nursery-sized options.
If you want something in a pot that comes from a bonsai place, or is further along in general, Wigert's from Florida is a great option and they have example photos on their website. They also have a rather large catalog of larger material that's already in a bonsai pot and well into refinement, if that's what you're looking for.
Perhaps someone else could chime in if there's another good resource I'm not familiar with. I know many people here have very impressive Bougainvillea bonsai, and I'm still quite new myself so I'm sure there are many other resources out there.