x0xsh0p.de Has kits, parts and a cool VCO mod.
rv0.be Get your x0xlarge CPU upgrade here. Also has panels and a few other parts.
abstraktinstruments.com If you need a non DIY anolog 303 solution the the avalon bassline is the way to go.
re-303.com Not a store yet but links over to a forum. User DinSync is working on getting a exact replica of a TB-303. Not for the faint of heart.