Check your phone book for Auto dismantlers or Pick-A-Part yards near you. Or check on
www.car-part.com, but I bet shipping on a fender would suck.
You will be able to use the fender piece from any Impreza (or Outback Sport) from 93 - 01.
You should have no problem finding one, but color-matching will not be so easy. Maybe take it to a cheapo paint-n-body shop for a "close enough" fix.