Well, it depends on how big you want to go...
The biggest plastic U-boat out there currently is the Revell Type VIIC in 1/72. Nice kit, but big (about 37 inches in length).
Revell then does a series of U-boats in several scales, the Type XXI in 1/144, the Type VII in 1/125, a Type 206 in 1/144.
Academy also does boats in various scales, and Nichimo does Type IX and Type VII boats in 1/200.
Mirage of Poland does a series of U-boats in 1/400 scale, and there's a few in 1/350 from various manufacturers.
Of course, I'm leaving out all of the nukes, attacks, and 'box scale' kits that are all over the map as far as scale is concerned.
Hope that helps,
Jeff Herne