I've been making my own custom tuner buttons for a long time. One of my favorites is Cocobolo, especially when it's made with a light colored sapwood cap:
I just bought this new piece of Amboyna Burl and I'm pretty sure I'm going to have a new favorite!:
This is 10 1/2" by 5" and 2 1/4' thick. I expect I'll get quite a few white capped Amboyna Burl tuner buttons out of this