Any small jobbing shop with a CNC milling machine would be able to cut a steel stamp for you.
Probably get it cheap if you mention its for your pinball machine just ive them plenty of time to do it and they would just squeeze it in when they get time.
Provide a piece of steel for them with the cad model and it wouldn't take them very long.
To stamp it in the caps, you would just heat up the stamp and press it into the cap and then paint in the grooves.

I'd prefer aluminium, it's so much faster to work with CNC. And doesn't take so much energy to heat up, if planning on heatstamp those logos.