www.SummitRacing.com is always a good place for performance parts. they specialize in muscle car parts (or are known for it anyway) but they have a decent amount of import/tuner parts now. air intake kits, exhaust cat-backs, brakes, springs, sway bars, ecu chips/tuning, pulleys, bushings, clutches, flywheels, shocks/struts, and even regular maintenance type things like spark plugs, filters, tires, etc.. and the prices aren't bad.
if you want TRD parts, I'd guess ebay is the best bet for that.