Not wanting to diss you nor your engine, but the 5S-FE is hardly a tunable engine (well, not the extent of a 1JZ-GTE lets say). You will always have trouble finding performance parts for it.
I had enough trouble finding performance parts for a 3VZ-FE when I had one. Buying a car with a standard 1JZ-GTE has opened up a great world of mods for me now... if only I could afford to do them (currently a student)