Justin, try
http://www.car-part.com for a transmission. You will not be able to use a transmission from an S10 or S15 unless it has the same diesel engine. 81-82 LUVs, 81-87 PUPs, and 86-87 Troopers are the vehicles that have the transmission you need. Now whether it is a 4WD or 2WD trans needed you can scratch looking for a trans from the Trooper since all were 4WDs anyway. Also, you have to look for a transmission special to the diesel engine because the gas engine transmission has the starter mounted on the opposite side. I would think car-part.com should be able to help you out though.