If you plan to keep the car for more than 1 year, by all means have the transmission repaired by a reputatable transmission shop (AAMCO etc.) or dealer. If you plan to fix the car, drive it for a short time then sell it or trade it, I would have a local transmission shop or a well established auto repair shop get a used transmission from a local wrecking yard, U-Pull-it, or similiar source. do not accept a replacement unless you can get a replacment warranty for at least 90 days. the shop should offer you a free or very low cost to replace the transmission the second time if needed. An alternative is to try to sell the car as-is on Craigslist or in a local paper, or approch a dealer about trading the car in as-is. the dealer may make you a good deal, but probably not.
