Just changing the back box is the cheapest way to upgrade your exhaust. I'm currently building one for less than £50 with a straight through muffler (basically looks like a big resonator) It does take a lot of time to do however, and some knowledge of how to weld. If you head to a garage that modifies cars, there's a good chance they'd be able to sort you out with a back box relatively cheap. An even simpler option is to do a resonator delete and keep the stock muffler.
If you don't want to do all that and just buy a pre-made muffler, any designed for the mk2/mk3 fabia, polo 6r/6c should fit fine (they all have the same chassis), but check the images as polo exhausts may have the hanger in a different location.