Extreme Supply has fantastic prices on closeout gear and will price match if you can find it cheaper. On a plus side for anyone in SoCal, they're located around Long Beach, so even using free shipping, I consistently receive my orders within 2 days of placing them.
There's also
JakeWilson.com. I've never actually used their street half, but I've recently started frequenting their dirt section:
RockyMountainATVMC.com. They've consistently had great prices and fast service for me. I can't imagine their service differs much between their dirt and street stores.
Oh, and while it's already been mentioned, I also adore STG. Incredibly useful site and they often have really nice sales
I've also used and been pleased with:
Cycle Gear (Of course. Although I use the physical store much more than I do the online site. Returning stuff in store is pleasantly effortless, but I've yet to have to online)
Motosport.com
And I don't have any personal experience with them, but there's also:
Kneedraggers.com
Revzilla.com
Bikebandit.com (I've heard these guys are slow though, but I don't know)
Fortunately (Or perhaps UNfortunately), I've yet to have a problem at any of these places, so I don't know how their customer service would hold up under difficulty.