My local Corolla Club, NC4 (Northern California Corolla Car Club) Has quite a few '03 members and they also have a site called 03Corolla.org or something along those lines.
This is what I see mostly: Stereo systems, Altis Grills, JDM headlights, clear corners, springs(eibach, progress, etc.), Shocks (KYB, Tociko), Rims (whatever), Body Kits, Anti-Sway bars (Hotchkis), Strut Tower bars (TRD or Ebay), CAIs (AEM or TRD), TRD Superchargers (haven't seen one in yet $3000) Cat-back exhaust (not sure who makes them for the '03s, Thermal R&D make a cat-back for 98-02), Mufflers (generic N1-style and TRD).
I think that's about it for mods I have seen on '03 Corollas.
EDIT:
Shit I didn't even see that you are in San Bruno, I go to school at night up at Skyline, here's the car-club site:
http://sfbay-corollas.dhs.org/
and the forum:
http://kenao.linuxppc.net/cali/forum/index.php
Check it out you can post there about modding your car and the '03 guys will fill you in.
Peace
Chris