While I was working with K&K nursery, I inquired about seed potatoes. Here's what the email response said:
"We will have Pontiac, Yukon gold and Kennebec potatoes. Our new potato source has numerous varieties but that has not been firmed up yet. Let me know what else I can do to help you."

Atwoods always has seed potatoes, and I don't know, but would expect, that Horn Seed would carry them?

Last year we purchased ours at K&K. The Kennebec are longer lasting than the Yukon Gold so we planted both. Considering it was a bad garden year everywhere, I was pleased I had Kennebec last thru November.

I've also seen loads of seed potatoes at Atwoods in Norman. Lowes & HD carry them also at the beginning of the season, but they're ususally priced much higher than afore-mentioned places.