I buy 1/2 and whole hides at Oregon leather here in Portland Ore. You have to order them through them (or elsewhere, but I been buyin from them for 20 years..) as they don't carry em on the prem. anymore. They sell rarely enough as to not warrant having a bunch on the tables getting handled by every snot nosed SCAer that cant afford them anyway.
They aint cheap, but I prefer quality to cheap.
A "cheaper" option and almost as nice is Moose. Bout a 3rd the money as for Buffalo and almost as supple. Not "cheap" by any means as quality goes, but some of my work goes BECAUSE it's buffalo. An odd affection Americans have with that particular critter... well, it's bits n pieces anyway.