You can buy a 4T60 transmission online from a number of rebuilders but expect to pay $1200 and many rebuilders require a core return, It is a bolt in swap but there are some new electrical connections to be made. Those transmissions were widely used by GM in Chevys, Pontiacs, Buick and Oldsmobiles in the late 80's and early 90's.
------------------
" 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.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "