They're getting pretty tough to find. Part number N-013-805.3 is NLA at the dealerships, and both of the local bug houses are sold out.
Good news is that GM cars of the 1980s used a similar part (GM PN 559563). I bought a couple at the local Chevy dealer. The GM part looks pretty close . . . a hollow crushable washer like the VW part, but copper-colored instead of silver. Also, the GM part appears just slightly larger at 1/2" inner diameter, compared to 12mm for the VW/Porsche part (about 0.25 mm difference). FWIW, the McMaster washer looks to be solid copper . . .
The Chevy parts man said that washer is also on the way out, but NAPA and Dorma both have a cross reference (NAPA is 704-1973, Dorman is 095-014). I haven't looked at either of these yet, but plan to in the future.
I'll try to remember to post a picture of the GM washer next to the VW part tomorrow (if I can remember where I put them).