What car is it for? Otherwise we could be sending you off in the wrong direction!
S15 clutch is pretty much the same as a S14 clutch. Its only the flywheel that is different on a 6 speed spec r! (The friction plate on a S15 clutch is unsprung because it has a dual mass flywheel, the S14 friction plate is a sprung item, it does not affect the car when fitting a 14 clutch to a standard 15 flywheel)
Spec s are the same as a s14 turbo clutch and flywheel.
A guy named jib on here is selling a flywheel for a 6speed box. (See forsale section) You will need new flywheel bolts though. Or contact Driftworks and they sell the same flywheel with everything needed.