Goodluck finding a decent muffler for a GD. there are a couple of online places (jdmconcept, passwordjdm, jdmyard [see the theme?]) that sell mufflers and full systems but pretty pricey. cheapest i found was JDMconcept [edit: seems they're gone ]

ebay might be your best bet, or wreckers or jp auction

let me know if you find anything coz i'd be interested