KC driver's door squeak

Over the years, my Titan has developed a squeak in the drivers door. It is much more pronounced when it's raining or really humid outside. I've tried the silicone grease on the weatherstripping which helped marginally but seems to be a band-aid at best.

• Is there a way to adjust the door to get a tighter seal? I've tried adjusting the striker and latch--didn't do anything. Perhaps the door is sagging--this truck is 9 years old with over 100K on it. Does that normally happen over time?
• Is it possible that the weatherstripping that is just worn out and lost its sponginess? It doesn't leak when it rains or anything.
• Where do you buy the weatherstripping. After an exhaustive search, I can't seem to find this part for sale anywhere online. It's the long continuous piece that goes around the entire door opening on the cab--both the door and suicide doors close against it.