I don't know of an independent breeder of Buffs in the U.S., but from what I heard Metzer Farms bought out most of the American Buff stock geese from Holderread's Waterfowl when they were thinning out their goose breeds. With Metzer's added Buffs from Holderread's, I am thinking about ordering a pair of goslings from Metzer next spring.
Poultry09 has an ad for two beautiful American Buff geese in the buy sell trade section on BYC if you can find a way to drive to Oregon to get them.
Also there are sometimes ads for Buff geese on Craigslist, you might check there once in a while if interested.
Good luck!
Faith
Edit:
Just from quickly browsing around, it looks like Silver Spring Waterfowl on BYC might have American Buff geese as well:
https://www.backyardchickens.com/t/628893/pics-of-buff-americans-please/10