Catalytic converters don't normally 'just go out'. Some diagnostic work needs to be done in order to determine the root cause for the failure. Often times we see car in the shop that have a blown head gasket(s) or excessive engine oil blow by that will contaminate the catalytic converter and cause it to stop working correctly.
Also, the problem may lie with the oxygen sensors.
What led you to the conclusion that the catalytic converter(s) are bad?