I haven't seen one for sale as a separate accessory. A service center would be able to order you one for a polisher they service. In the following link you can order the handle for a Makita 9227 for $8.47:
Makita 9227C Sander Parts : eReplacementparts.com
I'm not sure if that is the size handle you're looking for, but you get the idea now of how to search once you find the handle that will best fit your polisher.
As far as PC's and
UDM's go I find them much easier to use without a handle. I just cup my free hand over the end of the polisher.