Thanks to TSOALR, I have discovered This World's Greatest Retail Website. It's called Battlewagon Bits, and is worth a look.
As the name suggests, they sell GW bitz, so if you're looking for that one, special bit to finish off that luxurious conversion, or just something to glue to the side of your latest tank to make it look that little bit more spiffy, BWBits is for you. Their prices are outstanding too, as is their "Bits trade-in" scheme, whereby you can send them your discarded crap and earn credit to use in the store.
Whilst we're on the subject, I must give a quick bundle of kudos to the guys at TSOALR for their touching tribute to the fallen at Virginia Tech. Simple, yet beautiful.