Rockauto.com sells a reman. transmission with a 3-year warranty. It's $2414 plus a core charge of $1000. Shipping is $186, but use discount code "5387936748131651" and it'll take 5% off ($120). It'll probably cost you another $186 to ship the core back, so you're probably looking at $2670, which isn't that bad of a price.
Rockauto also has rebuild kits, which run from $134 to $560, depending on how much you want in your kit and brand.
For used transmissions, which I would be leery of considering all of the cross-contamination problems and the number of different valve bodies, you can try
www.car-parts.com.