Bathroom or the main vent? The main vent can be found pretty much anywhere, including Amazon. 14x14. The bathroom vent is specialty (9x9 I think), since they've pretty much standardized the sizes now to all be the size of the main vent.
I'm pretty sure this
is the bathroom vent, but you'd want to measure yours just in case. I replaced all the vents on my roof. Good idea anyway on these old trailers.