The U.S. has a "not-invented-here" and "buy American" mindset but I've long thought that there would be some value in procuring non-nuclear submarines in limited numbers for the U.S. Navy. The Germans make state of the art Diesel-electric submarines with air-independent propulsion (AIP) that can operate submerged without snorkeling for far longer than older Diesel boats. As does Japan.
The premier German submarine design/shipyard firm, HDW, has built Type 212A boats and related designs for a number of navies. The boats are much smaller than U.S. SSNs and have much smaller crews. They also cost much less to buy -- Somewhere in the $600 million range per boat versus the Virginia class SSNs at $3.6 billion each.
Check out some of the numbers:
-- 1,800 tons submerged with 2 decks (U.S. SSN: 8,700 tons with 3 decks)
-- Diesel-electric propulsion plus AIP for an endurance of 18 days without snorkeling; 20 knots max submerged (SSN: Faster, unlimited endurance)
-- 6 21" torpedo tubes (U.S. SSN 4 21" torpedo tubes with more reloads, plus 12-14 vertical launch tubes for cruise missiles)
-- Crew of 27 (Mixed-gender I think) (U.S. SSN crew of 135)
https://en.wikipedia.org/wiki/Type_212A_submarine
HDW has also designed larger boats for some other navies: For instance, the 218SG for Singapore's Navy (second photo) is larger than the 212A.
I presume that an HDW boat could be assembled or partially built in a U.S. yard, although that might impact the cost. I also presume that the Naval Reactors crowd would put up a hell of fight against the procurement of any non-nuclear submarine. The key would be that an SS would be bought in small numbers and useful for training U.S. Navy forces in ASW; it would NOT be used for high-speed transits or long deployments and the like. There is simply no substitute for an SSN for many functions and a conventional boat would not be procured in lieu of nuclear boats.