John
The taller reservoirs are available from most British parts suppliers, Moss Motors, Victoria British, Englishparts.com, LittleBritishCar Co.
Be prepared. You won't believe how expensive a 6" tall tin can costs
And if you're intent on being period correct, you'll need two tall ones and one short one.
Paul
ps: the short one can easily be made from a PVC glue can. Never could find a source for the taller can though