JC Whitney has them, PRICEY for sure at $308 but to each his own. We had one on our Holiday Rambler. It was a pain to install. We needed 3 hands to get it back on. I check my tire pressure every day and check the spare weekly when travelling. Taking the cover off was easy, putting it back on meant holding the fiberglass disc in place while aligning the stainless steel ring over the tire and then, making sure the disc didn't move, crawl down to latch the compression latch. Just aligning the stainless ring on the tire was a challenge, keeping the fiberglass disc in place (and straight so the HR logo was level)....UGG!!! I'll stick to my vinyl cover, just pull the bottom up, check the pressure and pull it back down.
Anyway, if you want one, here's a starting link:
http://www.jcwhitney.com/aries-tire-cover/p3044189.jcwx