You can get coated optical glass from Schneider Optics (Schneider's U.S. arm), cut and bevelled. For pieces in that size they will source it from their cinema/TV filter division. That's what I ended up going with in my Saunders 4550xl 4x5 carrier. In my 35mm carrier for the same enlarger I was able to use Schneider/B+W MRC "Multi Resistant Coating" glass cut from a clear B+W filter. As noted earlier, however, I can't see any difference in print quality compared with the non-coated glass that came with the carriers. And these modifications were not cheap.
Just curious Michael, what was the approximate cost of the 4x5 glass?