There are a few sources out there.
Moscow Hide & Fur shows one in stock for $15.00.
Sunrise Trading Post lists them, "when available," for $19.95.
Claw, Antler, and Hide in Custer, South Dakota lists bison horns for $15.00. I bought a nice, mature bull skull from their store when I was up there in 2006 for $125.00. It's a beauty, and I ended up giving it to my son. This outfit is near Custer State Park, which has a very successful bison management program. They probably have access to some good horns, but you may want to call them and ask.
Finally,
Sioux Trading Post has about the least expensive bison horns you'll find anywhere, but they appear to be out of stock. I have bought several
young bull horns from them for $6.00 apiece. These are suitable for small to medium size powder horns, but the hollow runs very close to the tip so you don't have much solid material there to carve for a spout. The last time I bought bison horns from Sioux Trading Post, they only had bison cow horns, which were quite small. They only cost $4.00, though. The folks who run this shop are very nice people, and Rapid City is right there in the heart of buffalo country. If you give them a call they may be able to help you find what you want.
Good luck!
Notchy Bob
"Should have kept the old ways just as much as I could, and the tradition that guarded us. Should have rode horses. Kept dogs."
from The Antelope Wife