Check out
www.subguru.com. There's a HUGE section of downloads and missions for Sub Command.
Definitely download SCX ! It's well worth the install and has a lot of really cool extras (like the ability to install new drivable subs like the Ohio class that is for download right above it on the site).
There are also some nice downloads right here at
www.subsim.com in the Downloads section (You don't need the 108 patch if you're using the version from the Naval Combat Pack). There's a couple of tutorial documents there and a bunch of missions for download.