I've had to repair two door regulators on my '01 Bonnie; the left rear and the right front.
I got lucky on the left rear because no retainers broke during disassembly. That wasn't the case on the front right door, though, because I managed to break the top right one. I didn't figure it would be a big problem getting a new retainer...was I ever wrong. The only place I could find them was at the Chevy dealer, and my local Chevy dealer prides itself in having the highest prices for OEM parts in the known universe.
The door panels use two different sizes of retainers, as far as I could figure out from looking at the dealer computer screen. I bought one of each size-- mostly to get the part numbers so I could get the retainers from a hopefully cheaper source on the net if I needed more.
Here'* photo of the two different retainers. The longer on is GM part number 25707329. The shorter one is part number 25704141 (by the way, the long one cost me nearly $5; the short one was $4.11--like I said, my local dealer prides itself...)
The retainers are snapped into their grabbers when you get them from GM. Flat out, getting them apart is not an easy task. However, I used a retainer removal tool to get them apart, as pictured below:
However, pressing the retainer against a piece of wood doesn't work very well, you need sometime tougher than that.