2002 Lexus ES 300 Catalytic Converter

Computer problem
2002 Lexus ES 300 6 cyl Automatic 187000 miles

What should I expect to pay for a new Catalytic Converter? Should I only take it to a dealer?

Any advise, my engine light code is that it's going out dont know the exact code, have just been told this twice from shop. I live in CA so I know it has to be done professionally.

Thanks
Corinne