Are you after a standard cat or a 200cpi Sport Cat?
There are many available from ebay but you do get what you pay for. I have had a Hoffmann 200cpi Sport Cat and it was very good. These are available from on-line autosport http://www.tuning.co.uk
The Hoffmann comes as a complete stainless steel downpipe inc. the cat. IIRC they are £379.
I currently have a HJS Sport Cat which is tip-top. These are available from Courtenay Sport.