There are two places I use to find out-of-print books of all sorts, not
just cookbooks; both are excellent sources. The first is:
http://www.alibris.com
And they can find you just about anything (their catchphrase is "Books you
never thought you'd find", and they really live up to it). The second is
not only online, but also has stores in several states. I'm fortunate in
that two of those stores are very near where I live, and I'd say more than
half of my cookbook library, I purchased from them. They carry not only
publishers' overstocks and out-of-print books, but they also buy and sell
used books, music (tapes, records, and CDs), computer software,and some
collectibles and odds and ends. They are:
http://www.halfpricebooks.com
And their name explains their pricing policy.
Enjoy!
Susannah