VW Beetle door skins are available for effective repair on your existing Volkswagen Beetle doors. However complete doors are only available from "parted out" VW Beetles.
If you purchase VW Beetle door from a parts recycler you might wish to inspect it to be sure that the interior framework of the door has integrity. You can add skins, lower plate, handle, door panels, even new Beetle door hinges. But the old Beetle door must have structural integrity.
Hemmings Motor News Magazine is a good source for specialists in parting out old Volkswagen Beetles (and their doors.) You can also work with a local "bone yard" that may be hooked into a network of other parts recyclers.
Ebay is a sometimes good source for old Beetle cars whose main purpose would be to supply yourself with recycle-able parts such as that door you need for restoring your Beetle.