Ok, he's a list for you.
Small Car (For your shift Kit)
Motorsports warehouse (This is our largest source for modds)
Subaru Parts (OEM Parts)
Online Service Manual
Level 10 (I believe these guy's use to sell upgrade trannies, looks like they mostly sell the parts now.)
ECUTune (This company speciallizes in our ECU's. They have there own section under MOD Mania, and longassname on this board is the main man there.)
That should give you a desent jump on things. There are a few other sources, by I tried to limit the list to the larger ones.