Just look in the yellow pages for anyone who is a dealer in "industrial compressed gasses." They will let you know where they get their tanks hydrotested and you can ship there..
BTW, If it is a Nitroduck tank -- whatever you do, don't send it in to Nitroduck to be hydrotested -- their customer service is AWFULL.. They have had my tank for well over a month, sent it back with a new reg (but still not hydrotested) and then I sent it back to them, and they STILL haven't hydrotested it yet...