woodencloud:

Plant a Yellow Transparent tree. This is an antique variety that has very sour apples, right up to the time they begin to turn yellow. Unless you have a very sour tooth, they will be sufficiently sour for you.

We use Yellow Transparent as our #1 applesauce variety, although it requires a little sugar, and it is also great for pies. Like many of the tart apples, it ripens very early -- around July 15 here in Northern Virginia -- and we pick them just before they begin to turn yellow, at which point they can begin to turn mushy.

I also have a much later-ripening sour apple of English origin called Foxwhelp, which is a real mouth-puckering experience. Foxwhelp is not so good for sauce, but it is great for blending in cider. You would probably like it right off the tree.

Older apple varieties like these are available at specialized nurseries around the country. You would have to do a little searching with your Google mechanism to find them. I think Cummins Nursery in NY State carries the Perrine type of Yellow Transparent, which is reported to be better than the original. You would be better off with a known grafted variety on a size-controlling rootstock like M-7 than trying to start a tree from seed, which is a real crapshoot. You would get fruit much sooner, and would know what you have.

Don Yellman, Great Falls, VA