There are some sellers who charge lots of money for replacement handles on the basis that they are for WEM, Marshall or Hiwatt amps.
However, the handles used by WEM and Hiwatt etc. are still available off the shelf.
I get mine from ESR Electronic Components Ltd.
Here is a link to the handles I use on amps such as the ER100, PA100 etc – Handles. At around £4 each I’ve yet to find them cheaper anywhere else.
The ESR order code for the correct model of strap handle is 407-025.