Try Car-Part.com. Those powertrains still seem to be available. Many owners of those old Cadillacs were seniors that drove them sparingly. Then they crunched them and they ended up in the bone yards. I often see them on Car-Part with low miles (50-70K Miles). Check them out. The swap isn't real difficult and it can be done on the cheap.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "