I have a 3speed automatic and I was wondering if they have a shift kit for those transmissions because I've been looking everywhere and can't find one. If you could help me out and tell me where to find one I'd really appreciate it. Thanks.

Transgo SK-125 and Superior K125 are "shift improver" kits for the TH125 that should firm up the shifts a little. All in all the TH125c is a good transmission though as far as durability. TV cable adjustment is critical to shifting, and the torque converter clutch solenoids are known to stick causing the car to stall when stopped. Solenoid can be replaced by taking the side pan off which on this transmission doesn't required popping out the axle though, or can just be unplugged.

I had been wondering the same thing myself, as my '99 Cavalier has a 3-speed behind the 2200 SFI. Do those shift kits have any long term effect on durability?

Anything that firms up the shift will generally increases the life of the transmission as you're reducing the slippage during gear changes. Transmission coolers also help increase the lifespan. Other than that make sure the fluid is at the proper level and is a nice red color. I believe in dropping the pan and changing filter/fluid more than others myself as transmission replacement or rebuild is generally pretty expensive.