If you dont have any reciepts, you could just make up a bill of sale for like $100 saying you purchased the trailer. They will charge you 5% tax on the sale price though. You could even use a reciept for a car or truck part that could go on a trailer. A friend gave me a small trailer that was on some property he bought, I bought some wheels/tires for it, went and said I built it and they gave me a plate for it.

I think the weight limit is something like 2k. If the trailer itself weighs 2k or less you wont need a title.