At my local GM dealer I'm fortunate to have two counterpersons who actually work on cars and are familiar with the catalog in part because the dealer also has a very active body repair shop. They match the new SKUs up to the old ones based on the length from hole to hole and thickness not exceeding that of the part they are replacing. Dimensions are in their online catalog.
Aftermarket works, but your problem might really be that if the counterperson isn't familiar with these parts, it's kind of a lot of effort to match up the SKUs for such small dollar items. Catch them when they are not real busy.
They might be a centimeter short, but they were the longest ones I could find. My original hangers may have stretched a bit over the years though. I think these will be fine, for as long as they last. Hopefully they don't crumble overnight.