Hello and welcome to the LOC.
I guess the usual places to look are Amazon and Ebay and perhaps Halfords.
Main thing is to get a breathable one that allows moisture out and a good fitting design, some of the cheap ones are little more than a sheet of polythene with a few fixings,the 1st high winds and away it goes!
Another idea if you are planning on long term standing is to get some silica gel packs to put inside the car which will absorb any moisture present.