If this is the part your talking about, i wouldn't really be getting my hopes up of getting a new separate rubber piece. From what i worked out when two of the windows on my old '82 seca did the same thing its, like a "special" press fit or something that holds the glass in the channel. All i did was just use some silastic to glue the glass back into the channels. Worked a treat
Edit - The passenger door side i used a channel from a '55, just had to cut it up a little fit. Never had any other problems after that for the 6-7 more years that i had the car.