Are you sure this was a Proper Nokia Authorised centre? A true Nokia repair Centre would provide a receipt for repair and they would also have done the repair under warranty as dust intrusion is covered by the handset warranty
I just recently bought a whole case from eBay for $15USD. It shipped from Hong Kong. It's obviously a fake, but it might have the parts you would need to replace it. When I get home in a few hours I can show you what came with the case.