As an alternative to ground glass, I have used textured polyester or polycarbonate as used for the graphic layers of membrane switches or labels.
Any local screen printer who does this sort of work would be able to supply some.
The company I work for (Parlex) has a factory in Newport, RI but does not do their own graphic printing. They use a company called Serigraphic in Boston but there may well be similar companies closer to you.
The product is either General Electric fine textured polycarbonate or Autotype F200 polyester (or equivalents). They are only 0.2mm thick but when held rigidly in a frame are quite robust and flat. We buy it in sheets 24" x 36"
If you are interested but can't find anything locally, I could post some to you.
Steve.