Update: I recommend buying metal pulleys. The steppers get hot, the pulleys can warp or just pull apart. If not then use http://www.thingiverse.com/thing:16627 instead.
I searched and didn't find one so I downloaded a model from Stock Drive. Then I used Nophead's pulley and a quick mashup of the two to give me an XL 10T pulley with a locking setscrew.
You can either use the final STL file, or if you want, grab the Nophead part , the .scad file, and the XL stl and compile it yourself.