Does anyone know where I can find seeds from old southern apple varieties for sale?

I'd talk to Fedco Nursery in Maine. They sell heirloom apple trees from the South. They might sell you seeds.

No apple seed will produce the same apple variety of the apple in which it came from... !  you will need a grafting or cutting from older mother tree to obtain the same genetics of an elder variety of specific apple...

I know how planting seeds vs. grafting/cloning works.  I'm trying to roll the genetic dice and see what I get. I'm interested in growing trees for livestock feed, not necessarily human consumption.

It's too late this year, but you might go for a road trip to Century Farm Orchards in North Carolina.  They specialize in old Southern apples.  He has an open house on Saturdays in November where both trees and heirloom apples can be purchased.

http://centuryfarmorchards.com/