Welcome. I believe you may be describing a typical 14"x14" vent. If so, it is a very common part.
This is just one source. Your local RV dealer/service centers may have in stock or you can find online several sources. Even your Walmart may have or you can find it at their online store, etc.
Search - roof vents - Camping World
This page has a selection of options over and above the standard vent 14"x14" roof vent as well.
If your vent isn't the typical 14"x14" then your search may be a bit more challenging.
Pictures of unit can be very helpful.