Lidl sell sesame seed covered buns, Tenglemann I think have small unseeded buns, and you can also get larger whopper sized sesame buns at
Kaufhof or
Karstadt or whatver its called at Munchener Freiheit.
As has been mentioned, they are a bit stale tasting, very dry. If you cut em in half, lightly grill the cut side, and then nuke them in the microwave for 10 seconds, it helps alot.
Kaufhof/Karstadt also sells Heinz gherkin relish, which when mixed with tommy sauce, some mayo and some onion, is pretty close to a big mac sauce.