I searched and searched on my Highlander branded trailer and never found one. I finally learned that these were manufactured in Canada and never had SNs applied in the first place. When I explained this to the DMV here in Virginia they couldn't have cared less.
There were several ways to apply for registration, and I chose the homemade route even though I bought it from the original owner and it was definitely not homemade. It was just simpler and easier that way, and cheaper. The three step process was to:
1) Go to the DMV and apply for a home made trailer permit. You pay the $$$, get the paperwork, and they provide you with a new aluminum VIN tag that you install.
2) Make an appointment with the Sheriff's office for them to inspect the trailer with newly installed VIN tag. Once they are satisfied they sign off on the paperwork.
3) Take the completed paperwork back to the DMV for approval and plates.
One thing I did to keep it convincing as a "homemade" trailer was to keep it in a slight state of disrepair with some obvious steps remaining for completion, all for show while the Sheriff looked it over. I told the sheriff that over the course of a year or so I got a piece from here and a piece from there, and I was finally at a point of being able to put it on the road, "in a few days."
They appreciated that and with a wink and a nod they were on their way and my trailer was legit.