Techno Toy Tuning is in Northern California. ATP Turbo is in Hayward, CA. As is Comp Turbo and Turbonetics.
I think that there's probably a larger market in the area for wholesalers that require a dealer to sell products to. Smaller parts that can be bought online usually don't require a dealer with a knowledge base for help. But for Turbo, ECU, and Engine parts, having a dealer to deal with locally is a really handy thing.
That said, Crower, AEM, BC (Brian Crower), and Eibach are also in Cali.
I was going to say, if you were dealing with euro's, RennTech is in Florida, and I was always under the impression that the middle-east has a larger following of benz's than here.