The DWM building in Karlruhe,
The Mauser museum and former company site in Oberndorf am Neckar.
The DWM site in Berlin.
The transport museum in Bad Oeynhausen, as it is attached to Holger Veh's 'teuto-arms' gun company.
http://www.teuto-arms.de
Also try to visit the shop of mr. Stabe in Bochum. He usually has some interesting lugers lying around.
http://www.wstabe.de/
Also CDS Ehrenreich in Neukirchen:
http://www.cds-ehrenreich.de/