Lost my limit screw! Shimano R8000 front mech - where to find?

I was adjusting one of the limit screw on my front derailleur (Ultegra R8000), and obviously slackened it too much, because it has fallen out.

Does anyone know:

a) Whether I can buy these as spare parts from Shimano? or
b) What the screw size & thread is so I find a 3rd party replacement?

I think the derailleur is already pretty close to the chain (which is why I turned the screw out so far), so it doesn't fall off in normal use, although shifting has become less precise.

Thanks!

John