Fibreglass arches are available from Scoobyclinic. But I would have concern about using these due to the bumper hanging on the rear edge :
http://www.scoobyclinic.com/shop/sty...hrepairkit.jpg
Alternativley trim up a pair of front arches for a metal solution. Front left for rear right and vica versa :
http://www.rs25.com/forums/f138/t171...st-repair.html