Vivienne, check out the info on CITES . Most nurseries will supply the certificate for you, but you will probably need to apply for permit from you State to see if they allow whatever it is you'll be bringing in. I think theres also a charge for this and that, and the permit could take up to 2 months to get, but check to make sure, it's different everywhere.
Thailand is known for Vanda alliance, but stay away from species, you'll never know if they were obtained legally. There are orchids everywhere overthere, you can also check out Thai orchid growers on the net, there are tons of them , most of them are in or around Bangkok. Let me know how it goes. Here's one of the links you can get some info from
www.fws.gov