Finding a nickel plated head might be a little tough.
Since ALL medium framed Colt's used the same basic parts, a rod head from any Colt Official Police, Officer's Model, old Trooper, 357, or Python will fit.
HOWEVER, older models used a different thread, so make sure you specify a later style threaded head.
You MAY have to buy a blued one, and have a local jeweler nickel plate it for you.
New rod heads can be bought from Gun Parts Corporation, who are the biggest Colt parts supplier other than the factory.
Gun Parts:
http://www.e-gunparts.com
You can also contact the Colt factory, and Pittsburgh Handgun Headquarters in Pittsburgh.
NOTE: Later Python's had a different ejector rod assembly, in which the rod and head are made in one piece.
These WILL NOT fit your revolver.
You MUST have just the rod head.