I saw some on eBay a few weeks ago, but I think they have gone now. There is a company called Lunns which sell these, but they are rather expensive. Your local plumbers merchant may be able to help.
Hello tamz, thanks for your replies in the past. The mixers I take off have had decent sized gaskets that didn’t need P.M. or s******e to keep out the showerhead left wedged behind the taps.Dean.