- Transport/logging truck parts departments like Mack, Whitestar, Kenworth etc.
- Marine supplies and parts.
- After markets suppliers of off road pickup truck supplies like tool boxes etc.
Those have been my best sources for the type of stuff you are looking for. A tip here, the marine places usually have the stuff in different colors so I used White.
The RV dealers parts departments up here are next to usless for that kind of stuff.
One place I asked about "U" type weather seal gave me a blank look and said they had no idea. When asked what do they do in their shop with RV's that need new stuff they said "We replace the door" Dah!
Best of luck