Do you already know yours is bad?
As far as I recall they're no longer available new from Toyota. Used it the only option and there are a number of places you may acquire one. As I would be planning on rebuilding regardless of where it came from I'd look locally for one which I could inspect and measure myself. If you want one that's already rebuilt check the classified ads on the various Supra sites (I think someone posted here with a freshly rebuilt one recently). You could also try contacting Aaron at
www.driftmotion.com to see if he has anything.
If you're planning on upgrading to forged pistons you may also use a GE block. You lose the oil squirters which are there to cool the stock cast pistons... not a big deal with forged though. You'll also need to have the block drilled for the oil return and supply for the turbo. If you go this route make sure you run a GTE oil pump and not the GE pump.