Coventrys sell them, well at least that's where I bought mine 3 years ago.

Instead of converting the Manual switch to an Auto, why not purchase a seperate Auto switch, and have both of them like I did? Manual mode is handy for MCs, drags while you're waiting in line for your turn and temps are just going up.

I had an 8psi Hobbs switch too, each bottle lasts around 3 weeks. Just put some water in each time you fill up at the servo, no sweat.