I agree that if you are using a standard wooden frame with a hinged back held in with metal spring bars, window glass from the hardware store will work just fine. Should it need to be custom cut to size, bring the frame with you to insure proper fit. If you have the store's "new Kid" cut your glass, it could be off by 1/8th or more. Cost should be $1.50 or less.
Another method is to get 1/2" thick plate glass from your local commercial glass dealer who does store windows. Have them grind the edges smooth to prevent cuts. Then lay your contact paper down on the enlarger easel, emulsion up. Place the film on it. Then lay the heavy piece of glass on the sandwich and the additional weight will squash everything down tight. The glass should be about an inch or so larger than the contact paper. This method is much faster for commercial production work. Cost: maybe five dollars or so.
Lastly, if you ever get into color printing and would like to eliminate the green tint of the glass, use "water-white" glass instead. It is clear, not green. Not necessary for b&w work. The cost for an 8x10 used to run about $15.