The system comes as components new so you should be able to order replacement cats without the rest of the components since its manufactured that was to begin with.
I would contact lethal performance and see if they can get just the cats for you.
Bassani sells them. Part number is 25MSC
Bring you wallet, they are $160 bucks each! Just had to spring on a set not too long ago. Amazon was the best price for sure unless you can score some used ones.