Why second hand? A new one wouldn't be all that expensive....still you could put a wanted ad on B&O, or maybe look on there, sometimes they come up for sale.
Unless the dinghy is of very low value, a secondhand cover, whether it is pvc or canvas will have degraded by UV exposure and will be good for only a short time, and therefore I reckon a poor purchase
I used to get mine from here, they are good, quick to deliver and (were) much cheaper than others
If you really want a tough long-lasting cover(but ugly and stiff to fold, then go for a black heavy weight plastic one- we used to use these in Datchet at the end of the Heathrow flight path and everything else, canvas and PVC, would rot in three seasons due to the muck chucked out by aircraft taking off. The black covers lasted much, much longer.
The dinghy belongs to my son who is still at University so I was being a cheapskate and hoping to get something on the cheap. But you are probably right and the best way forward would be to get a new cover. Thanks for the input.