Try the Bundesarchiv in Koblenz, Germany - I sent off for a set of plans for the KMS Gneisenau years ago, and they returned me a catalog from which I selected a series of individual deck plans, sections etc. They were not cheap, but printed on high quality transfer media at 1:100, i.e. several meters per roll. THESE WERE THE ACTUAL PLANS FROM WHICH THE ORIGINAL SHIP WAS CONSTRUCTED - you can't do better than that, and maybe the same can be found for U-boats.

I no longer have contact details, but shouldn't be too hard in this wired age.