Hi Patrick, I got it from Schneider Optics (the U.S. arm). If you are interested I can PM you the contact info. In fact I will need to contact them anyway because I need another piece of 4x5 glass.
For the 35mm carrier, I went all out with their MRC glass. They cut some rectangles for me out of B+W 007 MRC camera filters! You can't get better optical quality than that! One of the nice things about the MRC coating is how hard it is. Makes it extremely easy to clean and maintain.
For 4x5, MRC glass is not an option. Schneider Kreuznach does not currently put the MRC coating on glass large enough to make a 4x5 rectangle, and the Kreuznach office will not take custom orders for it. Probably just as well, since a 4x5 piece of MRC-coated glass would be well over $500. However, Schneider does offer coated optical filter glass in custom sizes from their film/cinema filter division. That's what I'm using.