Im looking for a Rear fender ext for a 1992 d350 cant find anywhere.

 

Should be available from dealer. They did a new production run in the past year, new ~$225 each side.

 

If you have pices of old ones buy fiberglass and rebuild your old ones.
I did and they came out great, used fiberglass mat and rebuilt them and then finished them down with sander,painted them look like new.
Claude