there are loads of unlocked iphones available on ebay, however, it is so simple to do it yourself.
Go to o2 or car phone warehouse, buy an iphone for £169 (contracts do NOT have to be purchased at the same time!)
Go home, download a program called ziphone from www . ziphone . org
If you are on a windows computer, plug in your iphone, itunes will then open saying are you ready to get started.
open ziphone, and click the "do all" button, or "unlock, jailbreak and register (I think thats what it says)"
wait about 3 minutes while the iphone scrolls some data.
when ziphone says complete, take out the sim tray (use a paperclip to push gently and firmly in the top hole of the iphone), insert your other sim.
Hey presto, your all done, without having to pay the extortionate prices that sellers on ebay are charging.
NB: once unlocked, the warranty is voided, this is whether you do it yourself, or purchase one of the overpriced ones on here. and do not install updates on itunes (installing itunes updates will result in the iphone being bricked (unuseable))
I hope I helped you guys.