Looking for the part # of the Test switch

Plenty of places online that sell switches, too; DigiKey and Mouser are two well-known (and reputable) companies that carry a large stock and sell to individuals. They are not necessarily the lowest cost suppliers, particularly when compared to surplus/closeout sorts of places, but sometimes the broader search isn't worth the effort to save a couple of dollars.

This is a snap-in momentary rocker switch. You need to find one with the same physical width and height (that of the opening) and suitable number and arrangement of poles. It may very well be a single pole single throw switch, on the + power line. Black color would be nice, too.

You probably already know, but you can get access to it by popping the display portion of the panel out with a screwdriver from the notch at the top, and then unscrewing the screws that hold the unit onto the cabinetry behind.

Try the Yellow pages
My YP has listing for 4 co.s and I use one of them regularly
I am sure they can help you out.

Any electronics store can sell you the correct momentary rocker switch. It doesn't have to be exactly the same brand and model.