I just finished replacing mine a couple days ago. I ordered a Lexan version from here...
Replacement RV Skylights from EZ Tops World Wide Inc.
It did take a LONG time to arrive. I think it's a pretty small business (possibly even 1 man show?), as when I called he mentioned having been sick. I am very happy with the product though. I went with white to cut down on UV rays and the accompanying heat. Lexan is supposedly unbreakable, but it does scratch much easier than the factory skylights.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)