It doesn't have to be from a 4-Door, any '66 Full Size B-Body Fender will work. The only differences is in Trim between some of the models. But you are right, they are hard to find in good condition or better.
I just sold a Drivers Side one last week, but even that one needed some repair work.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America