[GTP Dad]

Usually when you need to change a cat the cat will block up or break apart and will not pass the emissions test. When one blocks up you will lose performance and the car will try to accelerate and you will feel like you are sitting still. The engine will try to rev but it can't unlike the when the transmission is going bad and the engine revs but you sit still. You may also find that the cat will go bad and it will rattle. The contents will break up and you will hear a noise when you tap the cat lightly with a hammer or when you drive. If you are not lacking performance or you are not having problems with the emissions test then the cat is probably fine. With the newer cars with the O2 sensor behind the cat instead of being in front you will get a CEL indicating poor cat efficiency.
As far as finding a cat search the net or check with the a local parts store. High Flow cats are easily found from performance parts sites and there are even web sites that specialize in them. Simply search for High Flow Cats. Hope this helps.