I don't know what the cost of shipping internationally is, probably a few hundred dollars plus, but Colin has a good reputation here (and in UK of course) so factor it all in if you cannot find what you want in the USA, which by the way, will also cost a bit no matter what the origin/destination is.
An alternative is fitting a new 5-speed. Our member here, AlexD, just had this done with great success when his car was in for crash repair. The firm he used sells the Tremec and the kit to install it. A few hiccups along with way but nothing that wasn't solved quickly.
