Typically the vents are about 14" square. If that's the size you have then you can probably get any 14" vent to replace it with. Or, if you can find the brand name on the frame of it, you can Google for a replacement lid only.
Most of these type of parts are made by third-party companies, and aren't exclusive to any one brand of trailer. So you can just search any large RV supply company. There are a number of them on the web.
There were some 9" vents on early trailers, and for those there is only one replacement, which is of dubious quality (Elixir). But if you have the 14" size, you'll have lots to choose from. Even powered ones (Fantastic brand).