their are a few online bgb's kicking around. the install of the lsd is really simple, it's tearing down the tranny that is the harder part. You'll want a year specific LSD for your's if it's pre 93 (or at least in the case of cuscos) because at least on mine a 93+ didnt fit in a 91 Tranny.
Try Jay Marks toyota or a big oem parts supplier like that, the 91 from Japan had LSD in alot of their turbo models (gts) . Youll need a 91 lsd . Also try an after market co. like Phantom Grip. They make alot of lsd's for different makes of cars.Also check the racers forum here and see where and how they get their lsd's aftermarket.