I like the Zefal Spy mirror. Wraps around the lower end of the drop. Small and highly convex, good enough for seeing large objects that are coming up fairly close behind.
Nashbar shows a non-stock item but at least there's a good photo here. Lots of photos on Google but Nashbar was the best for having the recognizable site plus a good photo.
http://www.nashbar.com/bikes/Product...&top_category=