Hi mate welcome,
There are a couple of options, any forester mid section up to 2004 should fit not sure about the back box.
Classic impreza ones fit but require some welding to re position the hangers on the back box as the box is a slightly different shape and size,
Heywood & scott do forester cat back systems but they are pricey, and finally powerflow can make you one at one of their franchises nationwide. Not sure about costing though.
Or you could go the whole hog and get one custom made with new cats