I've never seen one, and honestly I doubt you will find one. I would imagine to be UL listed it would need the proper 20 plug on it (the one with the bent pin):
https://images.homedepot-static.com/...v4-64_1000.jpg
How many people do you know have a proper 20 amp outlet in their house? Let alone how many 20 amp extension cords have you seen?
Now don't tell anyone.... all the branch circuits in my house are 20amp. I plug the trailer into one of the barn outlets that has a nightlight on it, and used to charge the mower. So even though my dogbone is 15->50 amp, I can easily pull the full 20 amps through it. The wire on the dogbone is 10 gauge, all other wires are 12 gauge. So the weak part is the 15amp plug, but I'm guessing that for the brief periods it may draw over 15 (like when AC kicks on) there is enough mass to dissipate the heat.
Jim M.