In 1/192 scale (16 feet to the inch), you can get 5in./38s and 3in. 50s from Floating drydock, from a number of manufacturers, John Haynes, HR, etc.
Yankee Modelworks does a DDR Gearing in 1/350, the Dennis J. Buckley, which represents most of the DDR, pre-FRAM vessels.
If you don't have any luck with the 5in guns, I have a brass pattern I can loan you so you can cast your own.
I also have some additional parts, torpedo tubes, anchors, bitts and bollards, I can provide at a low cost.
Jeff Herne