Are you talkin sheetmetal or rubber mudflaps? Smart auto parts shops are stocking good rubber mudflaps this time of year......

Another idea - what they do in Vermont..... greasing dry undercarriage surfaces susceptible to rust. Get yerself a small can of axle grease then smear a thin layer all over rocker panels and wheel wells especially along the inside edges and on rust-prone surfaces underneath as far as you can reach. Maybe hit the door bottoms too, if they look like they rustin. You probably want to completely wash the fender wells and splashpans at 2 bit car wash and let them dry first for best effect.