Came across this problem myself. Even bought some NOS dampers, but the seals had gone and they didn't last very long.
As a result my 412 sedan currently runs with a super beetle one that had the end cut off and a Type 4 one professionally welded on in its place (damper kept in bucket of water while this was done to keep damper cool).
With just a little work there are a number of generic dampers that can work. I measured some out a few years back from the generic truck dampers available at Pep boys. A few of the 1" models juuuust barely fit.
Right now, I am running two type 4 dampers. They run "spoon style"...end to end...with a simple bracket holing them together. It works nicely for roads with lots of washboard. Ray