Had same problem with our Arctic Fox. That is why I went with a more common size. Northwoods wanted 3 weeks to get them to me. And full timing at the time. Hard to tell where we would be when they came in. Can't tell the difference looking up from the inside at a larger and more common size. Went thru regular RV part dealer, 2 days later was putting them on.
http://www.icondirect.com/skylights/ is what I ended up with. Or order direct. Amazon also has them I think. When I was putting them on, had a question and called their tech department, they gave great service and advice.
They recommended to put flat washer on the screw to dissipate stress compared to just screw, so far has worked out great. The caulking they send sure sucks to work with though. They gave reasons why for the sealant they use. Can't say I'm entirely sold on SB 120 caulking, but no leaks yet. Took more time getting roof cleaned up than putting them on.
2003 Majestic 23P... Northwest travel machine
2013 Arctic Fox 25W... Wife "doll house" for longer snowbird trips
2001 "The Mighty Dodge"... tow vehicle for "doll house"