Here is a motherload of high resolution scans of original Kriegsmarine charts. I don't exactly know how I got them, but I'm sure it's through a link here in this forum. Most likely the same as Jimbuna refers to.
Be carefull, it's 180 MB in size. You need to be able to open 7-zip compressed files. And the charts are different than you would see in the game map. Relative distances are wrong because of a different projection, and depths may not be the same either. The game map is a rectangular lattitude/longitude grid, with 120km per degree. But these charts are clearly different. Still, you can use the degree scales at the map edges to compare land features and city locations.
http://files.filefront.com/KM+Karten.../fileinfo.html