Lots of companies producing kits and hulls-
Germany- Engel and Norbert Bruggen
UK- OTW, Sheerline, MBD, Sarik Vacform.
USA- Caswell, Precision Pattern, Mike's Subworks, Scale Shipyard
Russia- Scale Ships
That's a few I can think of. Of course you have things like the Thundertiger Neptune, not a kit as such, at least not in the USA (available as a kit here in the England). I believe that Robbe are no longer manufacturing their line of submarine kits, or at least have greatly reduced the range on offer.