Where are you located JP? Finding a b13 tranny is not all that hard to tell you the truth. There are some available on this forum and also the original sr20forum.
You can use a 94-96, and 99 G20 lsd transmission. Also a 95-97 200sx Se-r transmission. The transmissions I just listed are direct swaps. The 00-02 P11 transmission and 00-01 B15 transmissions need some modifications.
If you want a sportier replacement look into getting the lsd transmission mated to the SR16VE.
You can use a 94-96, and 99 G20 lsd transmission. Also a 95-97 200sx Se-r transmission. The transmissions I just listed are direct swaps. The 00-02 P11 transmission and 00-01 B15 transmissions need some modifications.
If you want a sportier replacement look into getting the lsd transmission mated to the SR16VE.