Do a search on "Admiral's Lair" at MSN. He seems to have quite an extensive site on personal experience with Bits.
Shops:
www.towerhobbies.com (Very Reputable, but only sell MicroSizer gear - parts work on Bits though)
www.driftkingshop.com (Reputable, but mostly for hop-ups - limited selection of cars)
www.bitmod1.com (Don't know much, but they seem to have a wide selection of cars)
www.sigmaautomotive.com (Ditto on comments for bitmod1)
www.bitchar-gmotors.com (Don't know much. They have an auction site, but no one seems to be using it. A wide selection of cars, but they don't seem to have any stock for most models)