Graysonline have the white SGS III unlocked for $356.95 + free delivery. Warranty will have to go through them, which shouldn't be a problem (based on previous experience).
The blue and red variants are listed for more than $500, so hopefully this isn't an error. I've placed my order and received the confirmation and tax invoice emails, so hopefully it's legit!
Mod: back in stock as of 28/1 11 pm