I don't know about the junkyard question, but honestly if the weatherstripping is shot, it would be a good idea to replace it properly. These trucks are notorious for rust issues, and having water seep in, around, and under weather seals is a recipe for disaster.
You do not have to go with LMC, who price their stripping WAY too high in my opinion. I actually bought a LMC door strip, and then bought a kit from A1 Auto and the quality is identical, but the price is soo much better. Add on the fact that weatherstripping lasts for a long time, it's worth it to do it right the first time and then not have to worry about it for 15+ years.
https://www.1aauto.com/door-weathers...8/i/1awss00144
That's the kit I ultimately went with.