I did a google search on "antique window glass" and turned up a couple of suppliers - even got a hit in ebay, but they block that site here at work so I couldn't check it out.
But, if your looking for endorsements, sorry I can't help you.
Check with your local replacement window guy. A lot of old, wavy glass is sent to the dumpster as old windows are upgraded. I've acquired many nice pieces free for the hauling from guys that do this for a living. It's not only great for reproduction pieces with glass doors but if you get the "slightly flawed" stuff- it can be silvered for really good looking glass mirror! If it's too wavy- it has too much reflective distortion.
Thanks for all the great information. I especially like the idea of old window sashes from replacement companies. I looked on Craigs List and saw a few adds from people who have sashes with old glass. Already inquired about one. Thanks again for the help.