When I had to get a new cat on my Mk3 16v I took it to the local exhaust place, who ordered the stock part, which then I was told, didn't fit because VW did an odd fitting on the Mk3 for about a year and it's hard to get one to fit now.
So I took it to my local garage, and they got one made up/modified specially to fit my car and that cost over £300 in the end, which wasn't too bad.