Hi mate and welcome aboard
-To find some nuclear subs check this thread here:
http://www.subsim.com/radioroom/showthread.php?t=155535
You can also check here to find playable ships and subs too
http://www.subsim.com/radioroom/forumdisplay.php?f=219
Before you download make sure to read the instruction first because, not all of them are compatible with version 1.4
-I don't think these new subs and playable ships can be used in the campaign,