If you are looking for just a part of a lampholder then unfortunately its not as easy as replacing just one part. All manufacturers will make their own parts to fit their own lampholders so in the case below you cant just get the top part. Saying that of course if you were lucky enough to have the same make then it might just work.
Normally you would remove the complete lampholder and measure the diameter of the male thread. Its going to be 10mm or 1/2" in most cases, then you will need to replace the all the parts.