Plate glass is usable as a window for a scope. You do need to test out some glass at the shop to insure that it's not stressed (polariod test) and it's fairly homonogous in it's content. Gentle waves can be taken care of (probably from the polishing process not producing a truly flat surface) but quick discontinuities will be a problem.
Glass from the center of a sheet is usually the best place to look for the cleanest glass.
FWIW, I understand one of the SCT makers used to use plate glass for it's correctors.