Indeed finding good documentation and drawings are a challenge in scratchbuilding, but you are lucky in wanting to do a ship project. There are more ship drawing sets available than the scale drawings of all other modeling genre.
Never looked specifically for subs, but the two largest commercial sources of scale ship drawings are Taubman's Plans, and the Dromedary. Both are now owned by Loyalhanna Dockyard, but the drawing catalogs have seperate web sites. Do a Google search on each drawing brand.
Bluejacket also has some ship plans, but a very small number and I do not remember seeing a sub in their line.
There are also government agencies and museums that sell drawings, but I would start with Taubman and Dromedary.