Tein and Eibach both make lowering springs for it. Tein also makes a coilover kit for it, which our sponsor Enjuku Racing will carry.
Body mods will be few and far between.
Hawk makes performance brake pads for the P11, and I'm sure there's a set of drilled or slotted rotors available.
Lots of parts for a 91-01 Nissan Sentra 200SX are compatible. There's a cheap header available:
https://www.ebay.com/itm/For-91-01-Niss ... Sw9MZfxtsc
Rather than looking for one shop that sells something for a relatively unpopular car, you'll have to shop each component individually...
ALSO, make sure it's maintained 100%. New plugs, fresh injectors, and fluid flushes make a world of difference. No sense modding a car that's not already operating at peak efficiency.
The G20 / Sentra crew here know their stuff, so ask away!