I agree with Jon , these are great people to deal with . All my glass is purchased from them, in fact there is a dealer of glass 30km from me but I prefer to use these people.
Most emulsions will not create Newton rings
against plain glass. (Tech Pan did and there
may be others, but it's not an issue with the
Foma and Tri-X films I use.) But the film
base surely will. So, you can probably make
do with one sheet of plain glass, and one of
ANG, depending on your film of choice.