This person seems to manufacture them himself in Devon and might be a source though this listing is for the Rover V8.

https://www.ebay.co.uk/itm/BRASS-SUMP-PLUG-WITH-MAGNET-MG-TC-TD-TF-RV8-ROVER-V8/154032483025?hash=item23dd0d2ad1:g:FhIAAOSwUmhd8ptV