I looked many places to find upgrade pads for the rear, and had no luck. I ended up finding the Bendix performance pads at Advanced Auto. They work very well, if I remember they offer 3 versions of pads, an organic, replacement, and high performance. Since the front does 80% of the stopping you don't need real extreme pads for the rear.
I did call Hawk and they didn't offer pads for the rear, so I went with Hawks up front and the Bendix in the back.
Hope that helps!