You should be able to get a replacement cover through a Porsche dealer, or
maybe Pelican Parts or another parts supplier.  I don't think the cover is
particularly expensive.

I replaced the cover on my 1974 Targa and it worked out pretty well.  But
bear in mind it is a BITCH to make it fit properly.  The cover is made
slightly undersized so that when it's in place it stretches out tight.

If you are unable to find an original replacement any good auto upholstry
shop should be able to make one.  I think finding a replacement folding top
in good condition would be kind of expensive.

Alan