Flatbed or typical pickup bed, licensing laws etc are the same.
My swag would be there are some custom builders in Spokane. I can think of three here in the Seattle area minimum. If I include Everett and Tacoma, I can get to over a dozen. A local welder if there is one might be an option too.
Issue I have with some of the premade ones, they are what they are. If you want one 6" shorter or longer, no can do. I've seen some with the back corners angles a bit, so easier to jacknife a trailer. I've always custom done the ones I've had.
Marty