There's a number of different U-Boats out there.
Here's one,
http://www.subsim.com/radioroom/showthread.php?t=166898
Here's a link to get Tomi's Subs,
http://www.subsim.com/radioroom/showthread.php?t=173013
There's a playable Type XXIII somewhere also.
I couldn't find the link but I'm sure someone will get it for you.