Is the match a Level 3 or higher IPSC match? If it is, email and ask the USPSA President to send a request on your behalf to the Regional Director of that match. He will send off an email on official letterhead showing you are a competitor from the USA wanting to shoot their match. If their RD okays your attendance, they will apply for gun permits (if required) on your behalf.
Typically if its a Level 2 or lower they might not be too accommodating since applying for a single permit might not be feasible.
When we shot WS in Greece and the Australasia match in New Zealand, the USPSA President at that time submitted our request. Once approved they asked that the proper documents be submitted by strict deadlines. Those were Level 3 and higher matches where there was a large contingent of foreign competitors coming into their country to shoot a match. In those instances, they were more than willing to accept our requests.
There are other members here that have shot foreign matches more than I have. They will correct me and should be able to provide you more info.