Tramadol is not a controlled substance except for in Arkansas and Kentucky. (It's a schedule 4 controlled substance there). Since 2008 it also can't be dispensed legally on army bases.
If you try to buy tramadol from an international pharmacy it should be as easy as buying any other non-controlled medicine, such as advil.
But if you live in Arkansas or Kentucky and try to buy from an online pharmacy based in the US they will probably refuse to sell it to you.