Pennsylvania- Where do I get a form to have a VIN created in order to register an old boat trailer?

A Pa. website directed me to the police. I bought a titled 1972 boat with an untitled trailer. There is no title for the trailer and I can't find a VIN# anywhere. It has the original manufacturer markings on the trailer that correspond to the boat. The state trooper told me he went through the same thing, but got a form at a State Representative Office to have a VIN issued for his trailer. Does anyone know what form that is?