I don't think any of these parts are all that tough to find. I have several fender extensions for both sides of my '70. Unfortunately, I think the extra ones have small dings in them. I also have a set of remote mirrors (in unknown condition - somebody gave them to me and I don't plan on using them) but I doubt they are from a Chevelle.
Anyway, have you tried posting ads in the 'Parts Wanted' section? Or checking the 'Parts for Sale' section?
If you are interested in a less-than-perfect fender extension, I could check mine and see what condition they are really in.
If you are interested in my remote mirrors, I could dig them up from wherever they are buried in my garage and see what kinda shape they are in.
Good luck!