eBay be your best bet I got they mine I think you can use mk4 golf ones which are cheaper but not 100% sure on that
You got any pictures sounds like it might be a universal one because my original is not fastened like that
The Seat Leon 1M model arm rest also fits but the black leather is marginally paler than the S3. And to reiterate, make sure that the bolts, washers and mounting bracket are included. I think there are three bolts and three washers iirc...
This item number on eBay (just put it in the eBay search box: 330785831611) shows a picture with the bracket and bolts.