What is a reliable site to buy triavil without a prescription?

None. You cannot get any prescription medicine without a prescription, including online in the U.S. (either buy from U.S. Or live in U.S.). If you do, the vendor can be charged with a felony and you may be arrested by federal and state agents and charged with a felony. It is U.S. Law. Most of these medicines are prescriptions for good reason.
Triavil. This medication was discontinued by the manufacturer in November 2008. A physician could combine amitriptyline and perphenazine for similar effect.