Must be in the air, mine did the same thing about 2 weeks ago, I think that an auto glass store can probably get the channel, if not it would be a dealer item. A friend used to work at Autoglass plus and he helped me reattach mine, we used 2 small pieces of rubber at each end to center it in the channel so it wouldn't be crooked and used urethane for the middle section, rolled window up to a 2x4 wrapped with a towel so that any urethane that got pressed out wouldn't glue the window up to the top. My Channel was rusty, he said that the original method which just used a piece of rubber wouldn't work since the channel was rusted, told me if the window ever gets broken I'll need to get a new channel.
Good luck, let us know what a new channel costs you.
Caleb