I would suggest starting with the FSM, and here: http://www.teamfc3s.org/

87-91 RX7 Turbos II's came stock withe the 13BT. FD's had the 13B-REW. Cosmos had the 13B-RE. All are turbo 13Bs.

Are you wanting to buy a reman, or do you need additional parts, such as tranny driveshaft, rear end, etc? What are you using for engine management? What is it going in?
I have the FSM in PDF format, if you need it.