I have OEM covers for both our cars, but neither is waterproof and I'm not sure there even is such a thing. You'll get condensation trapped between the cover and the car and it won't be able to escape. Not good, a cover needs to be able to breathe.
Anyway, our covers are quite different materials. The NSX cover is cloth and fits very tightly, like a glove. The downside of this is I don't dare put it on except when the car is squeaky clean because it could scratch the finish. The SLK cover is a synthetic material and is a lot looser. Both have to be washed periodically.
After many years, my attitude as evolved and I don't use the covers nearly as much as I used to. Sorry I don't have any good tips where to find covers.