I would suggest you contact Paul Ambrose at Popham, who I believe is very highly thought of. His web site is:
http://www.special-flight.com/index.html
Otherwise in the US there is Budd Davisson in Phoenix who does ab initio Pitts conversion training:
http://www.airbum.com/
I do not think you need a visa for the US as you are not training for a licence or rating but really doing an extended check ride! To be on the safe side do a tail wheel conversion in the UK first.
I would have thought that some initial basic tailwheel training on something less expensive than a Pitts might be a good idea, e.g. a Cub.