I've placed an order with Gremlin Solutions (who have been fantastic so far with a very quick response even on a bank holiday).
I haven't been able to decide on a colour scheme for the buttons on the control panel. Black to keep it authentic, or coloured.
I've ordered a selection of buttons, 1 and 2-up buttons (black and white) and the smaller joystick with black shaft and top.
This should now conclude the parts I need to run and control the MAME cabinet. Now I'm going to sit back and wait for the parts to arrive. I'll post details of the working machine (sans cabinet) in the coming weeks.