I've found a couple useful sites while doing various mods to my Celica:
DDM Tuning 35W and 55W HID Kits
Great site for HID kits. What I like about their slim ballast kit is they dont need another power feed from the battery or relays mounted or anything, they are powered directly off of the plugs for the headlight bulbs. Just stick the ballasts down with double sided tape and you are in business. Not to mention the lifetime guarantee on the ballast. I installed the 35 watt 6000K kit in my celi a little over a year ago and still going strong. Great way to add HID without a lot of modifying.
PVC Speaker Adapters - CNC Machined - In Stock or Custom Made
Just received a pair of speaker adapters for the front doors on my celi. They seem to be well made and will make installing a 6.5" speaker in the door a lot cleaner. Depending on the design of the speaker, you may have to trim them a little, but for 10.00 a pair its a good deal.
Finally:
Celica Upgrades : 1994-1999 Toyota Celica Upgrades, Parts and Accessories
Found this site recently. They seem to have quite a few exterior appearance options for 6th gens. I havent done any business with them, but the site may be of interest to someone so there ya go.
As time goes by I'm doing quite a bit of modifying to my Celica. I am big on making sure all of my mods dont require any permanent changes to the car. So far I have installed HID's, grill mesh, DVD Head unit, smoked markers, short intake and other minor cosmetic changes. My future projects include a speaker system upgrade, LED dash conversion, rear disc brake conversion and a flowmaster exhaust. I may do some write ups for my projects if anyone is interested. Also, you can check out pictures of my car in the gallery.