All I could find is this:
SPF2687K Rear Cross-member Insert Enhancement Mounts
SPF1800K Rear Cross-member Supplement Washers
SPF1799K Rear Cross-member Supplement Washers
And they are all subframe insert kits… not replacement bushings.
Only thing I could find (on google) were the MMR Direct polyethelyne (not polyurethane) bushings for the S13/S14’s. It’s basically a hard plastic that won’t break, so you get stiffness of aluminum bushings but won’t get the squeaks/creaks that you normally would with the aluminum ones. However, they are just as hard and the ride will be just as harsh.