Catalytic Converter Woes

I recently got a check engine light, code P0420. Dealer said both cat converters need to be replaced. Out of warranty too. 90K miles. My mechanic said that code could be the O2 sensors, cats or mass air flow.

Anyone know where to get those cats aftermarket to avoid the 2K in OEM charges.

Is there something else I should be concerned about or are the cats just used up?