As of July 2008 it is illegal to buy or sell used catalytic converters.
You can buy an aftermarket catalytic converter as long as it is California ARB approved and used in the stock location.
It can be weld in or direct-fit as long as it was designed for that vehical.
It must have EO# on the shell of the converter.
Keep the receipt because ARB makes the manufacturer warranty them for 4 years.