I recently bought a N-O-S Black Knight backglass from Steve at Mayfair Amusements. There doesn't seem to be much online about Mayfair Amusements because Steve doesn't keep much of an online presence. This seems to be an antiquated approach to modern sales, but I guess he's not the only one in the pin/arcade hobby that runs things that way.
I'm just posting this as a recommendation for Mayfair. I was a bit leery of spending $395 (with trim package, shipping, and insurance/tracking) for a new glass without the standard backing of a credit card company or Paypal in my corner, however, everything went very smoothly. Due to the colder weather months, Steve recommended sending a 20% deposit to hold the glass, then make the rest of the payment early March for a mid-March delivery. I did just that and he shipped out my glass last week. I received it as promised, packaged very well, free of any problems whatsoever. The glass is beautiful and makes my restored machine pop. I've seen new glasses on Ebay selling for over $500, and some of them aren't factory mirrored repros, but the cheaper repros with grey paint instead of mirroring. Mine is the real deal with a Williams seal of authenticity and full mirroring. Big thanks to Steve. Although I don't usually enjoy spending almost $400 on just a backglass, this was an exception and he definitely followed through on his end.