The best suggestion is to go to us.gizmodo.com and do a search for iPhone 3G.
A week ago some SIM card hack was released which lets you "unlock" the 3G.
It involves mutilating your existing SIM but it does apparently work.
I have no idea where you can buy it! So search gizmodo then search the internet and ebay and order it if you can find it!
Otherwise you will have to have your 3G as a fancy iPod.
(or you could be really sneaky and buy a 3G and swap it for the US one and return it - they would prob not notice as long as its CPW/o2 but thats very bad! and I would never condone such behaviour!)