Where can I buy a used ls1 engine and automatic transmission ? ?

I wouldn't try to chase after trying to find a Chevy LS1 engine and transmission. If you want to do something fun, it can be done within your budget easily. I'd get a Borg-Warner T-45 5-speed, and find a late model Chevy 350 truck engine, with a four-bolt main bearing. If you do it up with a 400 stroker crank, you can get the cubes out to 383. Since your car is computer driven, a fuel injection system would do you better, and would be closer to a LS1. Holley makes great injection systems that truly are complete kits, especially with their Avenger Stealth-Ram kit. With that setup, depending how you do pistons, heads and exhaust, you could easily hit anywhere between 475-580 horsepower, and you would have something that not everybody else has.