HELP ? catalytic converter

I have a 95 mazda millenia at 102 kmiles. recently the performance of the car has been very bad. engine hesitates. it feels like no power during acceleration. i could not figure out what the problem was. then check engine light came on two weeks ago. i took it to a mazda dealer and i was told that one o2 sensor and both front catalytic converters needs to be replaced. I replaced the sensor for $250. but not the converters, which are more than $500 a piece. then, I took my millie to a couple of muffler stores. they advised me to take it back to the dealer because the converters are mounted into exhaust manifold. anybody knows a cheaper alternative to the dealer for replacing cat converters? thanks...

The catalytic converter should not affect the performance of the car. I just recently took my car to get a smog check and it failed the California 15mph test, but passed the 25 mph test. The car ran fine the smog checkers didn't even know the car was on. I ended up getting one of the catalytic converters changed and the car passed. The entire cost of checking my EGR, pre-test for smog and replacing the catalytic converter with a new one $176.XX. (US dollars) If you replaced the o2 sensor your car should be running fine. Total miles on my '95 Millenia 194K miles and still running strong. James