We were in Florida last year when we discovered a small water leak caused by a crack in the outer skylight.
I found an exact replacement at a local RV dealer, in stock, didn't even have to wait for it to be shipped. Ours is square shaped with one corner cut off.
You might take some measurements and try a local dealer.
Denny, Connie & Shadow (former barn cat made good)
2006 Tiffin Phaeton, Cat C7, Freightliner Chassis