Toyota Repair: Toyota Sienna Catalytic Converter, p0420 catalyst system efficiency below threshold bank 1, o reilly auto parts

Question
The check engine light has been coming on in my Sienna for several months. I have gone to O'Reilly auto parts and used their code checker and it gives me code P0420 "Catalyst system efficiency below threshold" (bank 1)
From what I have read, that is telling me the catalytic converter needs to be replaced. What I have been told about Toyota's is that you have to use their catalytic converter or the check light will not turn off. I was also told that the Sienna has 3 converters and you should replace them all. The estimate that I got was about $1700 and only about $200 of that was labor!
I am getting ready to sell the car, so what I need to know is the cheapest way to repair it so the light will no longer come on? If I can get away with repairing without Toyota parts, it would only cost about $300

Answer
If you are going to sell it or trade it in I would use the cheap converter, it should last well until after you sell it, the check engine light will not immediately come back on, even if you turn the light off by disconnecting the battery for about a minute, it usually takes a few days of driving before the computer sees a problem and turns the light back on.