First, as Skuits recommended, check warranty info. That fixed the code on my car last Feb and it hasn't come back since.
If it is indeed bad, check out Smith Brothers
Catalytic Converters, Smith Brothers. I called them last December and they quoted $350 for a complete rebuilt unit. (you send your old one back and get a core charge credited).
As much as people replace cats on Insights, I would not trust a used one.