Would LOVE to pick up a foot switch for my MG500 head, so I can properly dodge/burn with both hands and not worry about having to free up a hand to push the button!
They are expensive, but the connectors are proprietary so they are tough to find unless you encounter someone who has a head and controller that they are discarding, thus making the foot switch superfluous.
I looked for a used one for my Multigrade 400 system for quite a while before I bought mine from KHB. The switch itself is really robust - great quality. And I really like using it.
EDIT: I guess I should confirm - you are using it with the Ilford 500 controller? The foot switch actually connects to that controller - not the head itself.
Second hand Darkroom (a UK darkroom reseller) has one used for 29 GBP. I live in the US and have bought small items from them in the past - they are reliable.