they're available, but they've become more expensive than they used to be (but then again, what hasn't?) ...
watch the overall geometry when you make yours - I tried to make one in the geometry that I use for cow horns and it looks somehow unbalanced ... you want the handle portion longer than a cow horn one, in my opinion... there are some good horn websites out there...
bison seems more striated than cowhorn - so the grain of the horn is more pronounced ... this doesn't really matter all that much sine (as the horn is already black) you won't scrim it after all ...
bison softens using the hot veggie oil method, just like cowhorn ... don't bother with boiling water - the water doesn't get hot enough to 'get 'er done,' and all you end up with is a clean horn in the same shape, and a bunch of funky, nasty smelling water.
good luck with your project, and
make good smoke!