You can have a new "D" frame hammer fitted.
This is a gunsmith job since the hammers typically need a t least some fitting and adjustment.
Good sources for installation would be:
Pittsburgh Handgun Headquarters
1330 Center Ave.
Pittsburgh, PA 15229
(412) 766-6100
Cylinder & Slide, Inc.
245 E. 4th Street
Fremont, NE 68026
(402) 721-4277
http://www.cylinder-slide.com/
C&S will be slow and rather expensive, but they do good work.
I'm not sure Pittsburgh will do hammer bob jobs, but for price and turnaround, they are very good.
Of course, it's VERY possible Colt themselves will do it, and they would be my first choice.
For a replacement hammer, contact:
http://www.jackfirstgun.com/ (Contact them FIRST, their parts are usually new)
http://www.e-gunparts.com/
Note that a hammer from a Detective Special, Agent, or Cobra will fit since these are interchangeable on those models.