Chad, are you talking about the control faceplate, or a logo faceplate? If it's the control faceplate, there are a few places, including Ebay, but you have to be specific about what you want, because the hole patterns are different for different amps. If you just want the faceplate logo, then Fender has those.
Maybe @dougk or @telecaster911 might know somebody who knows somebody. That's all I can think of as far as that oddity.Musicmaster?...I doubt you'll find one.
There are a few sources for bootleg Fender faceplates...an eBay front for an amp supply shop; Tube Amp Doctor (won't ship faceplates directly to U.S. customers); a banned eBay seller who now operates a small website; etc., but they are are for much more "popular" models.
Fender faceplates require screen-printing, you have to get someone to laser cut and polish the faceplate and then someone to silkscreen on the blue paint and then separately silk screen on the black paint. All of the custom faceplate makers I know of only do laser-engraved plates
Plus, there was no historic blackface Musicmaster.
I can knock something out of pickguard material for ya! Probably too thick at approx 0.115". I'd guess the original is probably 0.032" or thereabouts.