If you need the mounting flange as well, that's a separate bit. As is the threaded hollow rod that screws into the lamp holder and mounting flange.
There seem to be two different screw threads in common use on these lamp holders, and there's a reducing bush available to convert from one to the other.
Strangely enough (but it won't help you) I find my local hardware shop on the high street keeps a stock of the threaded rod, bushes and mounting flanges etc.