I can't help with where to buy one but you can search a company called Calmark who makes one out of a breathable material.
I was going to buy one for my 30IOK but the dealer recomended that I do not. He stated they have experienced more issues becuase of a cover then issues with campers with out. Stated that the covers are heavy and can be hard to put on, creates unneeded traffic on your roof and breakage of vents and antennas when pulling on and off if not very careful. He also stated he has seen mold and rust issues due to poor ventilation, damaged surfaces from poorly installed covers that flapped in the wind putting marks in the finish. He also advised of a couple trailers that had been covered where the decals started to peal, was clear it may have not been the cover but had plenty of trailers with no issues that had not been covered of the same year with no issues.
He did state that an AC cover was not a bad investment, as well covering the hot water and fridge vents with plastic bags to keep snow out was a good idea. Advised to coat the RV well with wax and to not clean the snow off the roof.
Only other thing he offered if I did decide to buy one, uses tennis balls cut to cover the down spouts. This would give rounded edges instead of sharp pints that can tear the cover or snap the spout if caught and pulled on.
I think his recomendation makes sense for where I live, cooler damper climit. If I lived somewhere it was hot and dry, keeping the sun off it may hold more value.
2015 Sandpiper 30IOK
2014 Ram 6.7 CC DRW
2006 Charger RT