Depends on what you want to do...comfortable stock-ish ride? auto x? track? drift? show?

RacingBeat.com is a pretty good start for everything

Mazdatrix.com has a lot of stock replacement and repair items in addition to aftermarket

Corksport.com has a lot of pretty cool performance parts as well

Ground-Control has a pretty trick suspension kit that you can upgrade as your needs dictate, might wanna check them out if you think you may one day want better performance but don't want to shell out huge cash now.