Bald Bouncer and whitedog are the 2 main suppliers that I have heard of. Personally I use bald bouncer, but I think he supplies with the mods pre-installed, but not the hdd`s. You can email him via the web site and he can normally help or suggest an alternative.
PS: One of my friends got a xbox with mod, and upgraded HDD from ebay. Might be worth checking out the auctions.