I would'nt be surprised if you find out you need to replace the entire regulator. I think rather than tracking down that channel that holds the glass, I'd just order a new regulator(which comes with that channel attached).
It will be no fun to trackdown, and buy, and install that channel to an old regulator,... only to find out that the whole regulator needs replacing.
You can get a new regulator, with the hardware kit for under $50.
We replaced one regulator in my car last year. Once I looked inside the doors, I decided to order the regulator for the other side, and all new window Channels with the weather strips. Gonna install that stuff if and when the weather warms up.
I also decided that since I was gonna have all that stuff apart, I may as well replace the original scratched, and pitted door glass too.
Like you said once you try to fix one thing, 2 or 3 additional problems always seem to pop up.
So look the area over thoroughly, and order everything so you can tear it apart one time and fix it all in one shot.
Oh yeah,...while I have both doors apart it will be a good opportunity to install some sound deadening material in the doors.