Check German Ebay (ebay.de) or Egun.de for old scopes and search for zielfernrohr (scope). The repro Zielvier's are like $600 now anyway so you might as well go original. Make sure you see a pic of the bottom of the scope and that there is no integral scope rail on the bottom. If there is, you won't be able to mount the scope without either a custom claw mount or milling the rail off. Neither option is good IMO. Pre-war Zielvier's and Zielsech's are ideal.
If nothing else, it will give you a chance to brush up on your german! You might want to try a WTB ad in the EE also. Lots of guys have done these projects and that way you won't have to drill yours.