What type of Cat should I get for my 2000 Miata

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03 Silver Protege5
My primary cat has gone bad, car is running massively rich with a CEL. The code is for a catalytic converter so I'm gonna go ahead and replace the primary to see if that fixes the problem. How difficult is this to do for a 2000 Miata and where is the best place to get a cat?
 
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