Boot bulb for the s-type 1968 jag

Where can I get a Mercury boot bulb or equivalent for my S-Type 1968 3.4 jag please?

Two of the major US suppliers do not show this item but I can add that I have replaced mine at least once and it is still a bit recalcitrant, needing a sharp bump with the fist to come on. And keep in mind they only work when the side or head lamps are on.

Mercury is way out of favor these days.
P.

Thank you for your help.
Seems daft, especially with a dynamo, that the side lights should be on - encouraging the battery to go flat - I don’t need this in the dark!
I will try another way.
Glynis