My Alberta vehicle and driver registry suggested that they would give me a trailer plate once the selling owner faxed me a bill of sale, which they did. I also had the selling owner contact the original owner to supply a bill of sale (notarized) so that I had a chain of title crossing into Canada.

That did not prevent certain Canada Customs officers from throwing a hissy fit. To their great disappointment the VIN for the plate matched the VIN on the trailer.

Oh yes, the plate on the trailer came from a souvenir shop. The myth was that it was "agricultural equipment" not needing a plate. Will your friendly highway trooper swallow that or call in a flatbed?

Some states have trailer titles and others don't. Some states offer in transit permits and others make it difficult, if not practically impossible, for private parties.

If you do get an in transit permit, it's generally good anywhere in the US and Canada.

If PA won't give you one, maybe a bordering state would.

Moving from Ontario with a newly acquired car on a 10 day temporary permit, I had to get another temporary permit in Manitoba on the way to Alberta.

Good Luck!