Try Caloric Color, Garlfield NJ, 973-471-4748, as far as I know they don't have a website--
I've dealt with June over the years, she probably still works there but not sure, it's been 3 years since I bought from them.
They cast the stuff themselves, it's beautiful stuff-
I buy this red material to make LED digital display faceplates out of for a customer--
I just found this leftover piece an test engraved it--
RC2.jpg
the back side-
RC3.jpg
--quite obvious it's NOT extruded! It passed the engraving test, if the raw edges aren't enough of a giveaway!
The matte is subtle but effective. Beyond the raw edges, I've never found a blemish.
I tried to take and end-shot but couldn't get a decent focus. My digital calipers say it's .0605" thick...
I bought some gray matte once, same nice quality. Note it'll cost you more than Plexiglas
Hopefully they can help you out...
--edit-- as for getting 'white' engraving on black, or what I got on this red, or any color other than clear, a 'frosty' version of the base color is usually all you can hope for, like I got on this red. Extruded hit with a laser produces a melted-clear finish that's anything BUT 'frosty'. I say this because the black you have, may indeed be cast, especially since the ebay ad states that it is...