Doner car, or...Fabricate what you need from sheet metal. Or, if you have a collision shop supply store near you, you can purchase "Universal" lower door skin panels. They have a folded lower edge already in place. Just have to measure and cut to appropriate size...
Melvins Classic Ford Parts sells the universal lower door skins.... http://www.parts123.com/parts123/yb.dll?parta~partsort~51~cadffejo