Our requirements are rather unique, which makes outsourcing subs a bit of a problem. Most SSKs are fairly short ranged, which isn't a problem if you have a small coastline (90% of the world), or use them as the last layer of defense behind screens of SSNs (Russia). The "U" class subs we bought from the UK were much longer ranged than most SSKs, since they were essentially nuclear submarine hulls with diesel engines. They are somewhat larger and more voluminous(?) than other SSKs.
I had an article from the old CDQ, which I can't locate right now, but the proposal was to take the Italian FOCA sub plan and expand it to make real sea going subs. To explain, the FOCA is built from a series of large diameter steel pipes which are bent into circles, then welded together. (Picture a stack of tires in the corner of the garage to get the idea). The circular cross section of the pipes makes it quite strong, and the internal volume of the pipes is used as fuel tankage and to store high pressure oxygen. The fuel and oxygen feed into a normal diesel engine for AIP when running underwater. (presumably it uses outside air when surfaced). Italian navy FOCAs are very small machines designed to bring teams of 4-6 frogmen close to the target. If we go to bigger sizes and more pipes, eventually we could build a sub the size of a Victoria class sub (or the Kursk, for that matter), with transoceanic range.
Obviously there is a lot more to this, I would have to guess a "real" super FOCA would have a double hull like the Russians used to accommodate useful items like torpedoes and towed arrays. The moral of this story is we need to get off our butts and start cutting metal if we ever "hope" to gain much needed submarine capabilities and experience. Yet another item to bring to the table,