I assume you're talking about the rubber 'bed' that the glass sits in. The proper answer is, our vendors sell a product that fits inside the channel. Here's a link to Mac's but they aren't the only ones who stock this part:
MY solution is different because I'm cheap. I mean, frugal. First, I make sure the channel isn't rusted. If it is, buy new channel. Then I use rubber that resembles inner tube with glue that is used for setting Formica. My original 'rubbers' had an apron that simply hung down on the outside to divert rain water away from the motor and mechanisms. So, I glue the channel, stick the rubber in and immediately insert the glass, all while the glue is slick. I trim the inside edge flush with the channel but I leave about 4" hang down on the outside.
The box stores used to sell a rubber product for making ponds that resembled inner tube. I bought a couple yards and have been eating off of the roll for years. The rubber's thickness cannot be more than 1/16". Oh, and make sure your glass is CLEAN!!! Glue will not stick to grease or oil residue. I use lacquer thinner, NOT paint thinner. Acetone works too. Rub it in and use a dry clean cloth to wipe it off. Do this a few times.
For vent glass, I slowly heat the glass with propane. Eventually, the surrounding butyl rubber will start to melt. Simply pull the glass out in one piece. Clean the channel and set your glass in black urethane. Trim with a razor blade the next day. - Dave
Edit: I see tee-bird.com wants $3.00 and they call it B21478A. Talk with Bill or Ellis and tell them I sent you.