Where to find new switch for instrument panel test switch?

Grainger.

Chris's switch looks like the "official" correct replacement.

That's a standard snap-in momentary SPST rocker switch (normally open, not normally closed; in other words, the commonest case). Find one of the same dimensions (you may have to remove the existing one to measure the hole) and an appropriate color and you should be good to go.

Mouser, Digi-Key, Jameco, Amazon, Ebay, and many other places would probably have one available of the right description. Probably you could find one for less money than the official version, although minimum orders or shipping and handling from some suppliers may eat up much of that savings.