Cow horn is the "proper" stuff, if you're interested in making traditional English warbows. Buffalo horn is ok, but it has a different structure to cow horn, which means that if you're making sidenocks (as you should be
) it can have a tendency to split at the heavier weights. Draw weights of about 120lb or below and it doesn't make much difference other than for authenticity.
Remember that cow horn is a waste product of all sorts of processes, from farming to abattoirs so if you ask around you'll probably come across a good source somewhere. Failing that, eBay always has cow horn tips.
Once you find a good source of cow horn you won't bother with buffalo again, that's for sure!