exoticblanks.com has some (sometimes)
lazerlinez has some (i think)
and of course psi, csusa, and pennstate, rockler, and woodcraft.
If you are looking for really good top grade stuff, I think mrburls has some of the best quality burls.
How much are you looking for? I have my private stash of exceptional blanks. Here is a pic of one of my latest out of said private stash. Buckeye is one of my favorites so I have many slabs awaiting the bandsaw and stabilization process.