If you dont manage to get a VW reservoir you can easily fit an aftermarket one as found at places like demon tweeks, car builder solutions or Merlin Motorsports (where I got mine). You will need a small reservoir, some blue braided brake hose, a t piece and some barbed fittings to push into the rubbers in the Master cylinder.
i did this for my mates split as he had the same problem. still cost him £50 so not cheap but better than no brakes!
another option is to plastic weld up your old one!
EDIT:just re-read and see its the lower reservoir - in that case just do away with it and replace it with barbed fittings, and pipe thru a t piece to your upper res. in the cab.