as a glass blower, the things i would look for in a period peice of glass would be seed bubbles, a greenish tint, inconsistant thickness, and a size no more than 12 inches. and 12 inch glass would be very expensive.the glass should be a soda lime glass, that is to say, somewhat soft to the touch, it feels slightly slippery, and softish when compaired to modern glass.
the best place to get a replacement pannel would be a glass blowing facility. i happen to have access to one and the knowledge to make said glass. if you send me the demensions you are looking at then i can see what it would require to make it.