looking for first (apart of the header) or secondary (behind the O2 sensor)?
If it's the first, sorry, your only choice is to get from the dealer, or have a new one welded in place. Fabspeed looked at my set-up and determined they could make a custom set of headers with a free-er flowing header design and a higher flowing cat, but it's not cheap. $2000 to modify your existing headers, or $4000 for a completely new set-up.
If you're looking for the secondary cat, don't bother with it, just replace it with a bypass pipe. Still passes emissions (if cat is behind O2 sensor, it's not required), and no check engine light problems. $1400 and weorth every penny for the extra punch low end and better sound! http://www.fabspeed.com/maserati.html
I have my stock middle cat's available if you want them.