The CoPilot works well as an iPad briefcase.
(Oops -- didn't realize you were looking for a bag for a 13" computer.)
eBay. According to the Introducing the Cadet Briefcase thread this size Cadet will fit the 13" MacBook Pro, but not with a cache, I'll quote one of the posters, ShineAgain, since TapaTalk won't allow links to individual posts, and the thread is quite long:
I have the smaller Cadet, and my 13" MacBook Pro fits in it just fine. It won't fit; however, in a sleeve. I've hauled the thing back and forth to work without any problems. It's padded on the front, back and bottom, just doesn't offer much protection on the sides. I also carry my iPad 2 in the sleeve right next to the MBP, and there's plenty of room and protection for my comfort.