I'm not sure of where to go to get the best deal, but here's what to look for since it's all that readily available for the 99 I30Eibach, H&R and Suspension Techniques all make springs for the car. It'll be listed as a 4th gen maxima but it's the same thing. The place most people seem to talk about is:
http://www.cattmanperformance.com/
Tokico(also OEM), KYB and Koni all make aftermarket struts and shocks. Koni probably make the best shocks but if you get the adjustables it's a real pain to adjust the rears. You have to dissamble the rear suspension to adjust. The front is easy of course. I also believe Tokico makes an adjustable that is easily tunable, but quality might be sacrificed.
For body kits the only thing I'm aware of is the side panels from Maxima body kits can be used on the I30 and spoilers as well. A cefiro conversion for the front end is the only other thing I've heard of. This doesn't mean that there's not anything else available. I hope this helps get you pointed in the right direction.
Welcome to NICO. It's good to have more I30 owners on board!