Welcome! The 2 main suppliers are Intense-Racing.com
(being the leader) and ZZPerformance.com
. One car to have a look at (which is basically the same as yours, just modded up pretty damn good) is Rogue'* 2000 SSEi
These cars are very easy to mod and the aftermarket is very abundant. I wouldn't buy a PCM from anyone but Intense. With the fastest bonnevilles in the world, they pretty much know what they're doing. As far as exhaust mods go, they would be down the line of modifications. You have started out pretty right on. The next 2 things I would buy if I were you would be a modular 3.4" pulley and an Intense PCM.