Welcome to the forum.
If you don't find one at a salvage yard, you can always order one from a place like Icon. I once talked to an Icon rep at a RV show where they had a booth and they told me that they can usually match the size of any RV skylight.
They can give you support on how to remove and install the new one and they have a video posted on their site on installations instructions (attached below). They are a Canadian company.
Icon Direct ...
go HERE for website
(204) 325-1081 or (888) 362-4266
Also take a look at EZ Tops. They have dozens of sizes ...
go HERE for website and to see sizes
(877) 433-4568
Replacement skylights are made of Lexan polycarbonate and are now almost indestructible.
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