Besides checking with every Cadillac dealer (VWagons don't stay on dealer lots very long), ask if they know any one with a VWagon that might be interested in selling.
Check the auction sites, ebay, etc. - VWagons don't stay there long either.
"Bring A Trailer" might have something. When I get ready to sell my VWagon, I'm going to list it on BAT.
Go to local coffee-n-cars shows and ask around.
From what I hear, manual VWagons are pretty rare - less than 20% of production. Good luck with your search for one.