We got ours at Camping World, but I don't see them there right now.
Found this one on Amazon, our second home for RV supplies!
It's the same size as the ones we got. 60" X 75" or 74" is the closest we've ever found. Because we're folding up the beds, the extra width means we don't have to take them off every time. We've also got dogs, so my wife also puts a sheet over the cover.