I've never seen an aftermarket alignment slug like you're refering to either. I think the best place to get one is a junk door.
I have had some experience with door alignment. In fact, I've never had a replacement door fit correctly without some realignment. There is no factory adjustment provided. I think the doors were fited to the cabs at the factory using the information that 46forme refers to in the GMC manual. I also have had 2 cabs that had a crack in the firewall right above the toe board which allowed the cabs to sink. Of course, the wood blocks and rubber pads that support the cab all need to be in good condition as well as the door pins.
