I needed to replace the crumbling window channels in my 88 Wagon, and looked in the usual places for a replacement. There are a couple sources typically mentioned here, but I couldn't find a source with 90 degree molded corners. Some were sold by the foot, and you would need to cut a corner yourself by trimming some material.
I found a good substitute seal from a Jeep Wrangler. The TJ wranglers from 1998-2006 sometimes had full steel doors with roll-up windows. These channels have hard molded corners at the front corner, but the channel can simply be flipped. I ordered a set of 2 from RockAuto for about $25, the Fairchild KD1012 and they work well. You would need 2 kits (4 pieces) to do all the doors on a wagon.
Note that the seals are exactly the same width on the Jeep and the AMC's, but the Jeep seals are a little shallower/shorter. In other words, the AMC channels (the steel doors) are a little deeper than the Jeep's. On my wagon the seals originally had about 1/8" visible to the outside when the window was closed. The Jeep seals are exactly flush with the steel door so nothing is visible from the outside. There is no functional difference and I'm very happy with the fit and the price.