The 92 like i said will interchange with the 318 tranny. Firstly,i would check http://www.bimmerforums.com Youll find one on there for sure, but youll most likely have to arrange for shipping. Start by searching the for sale forums, if you dont turn up anything make a post in the wanted section.
Another good one is http://www.car-part.com - which is an awesome resource for just about anything but again, shipping will be an issue but i have found that they are fair. As for price, Id say anywhere from 2-350 bucks would be fair for a low(er) mileage trans. I have one that i pulled from a 92, but it is higher mileage (180k) and the car wasnt drivable so i have no way of knowing if it doesnt have the same problem and really wouldnt feel too confident in selling it.
If you cant turn up anything, shoot me a pm. Ill be able to turn up something.
Justin