Dealing with mechanics
If you're going to take it to a mechanic anyway, you are probably better off letting that same mechanic do the diagnosis and obtaining the correct part(s). Bringing parts and a diagnosis to a mechanic is likely to get as warm a reception as bringing your own eggs and raw bacon to a restaurant and asking them to cook you breakfast.
Doing it yourself
However, replacing an O2 sensor isn't usually that difficult. The first thing to check for is actually leaks in the catalytic converter or the exhaust pipes and headers. A leak ahead of both O2 sensors will "look" to the computer like this problem and replacing the cat or sensors will just waste money on the wrong fix.
If you have access to an oscilloscope, you can look at the voltage levels on both the front sensor (pre-cat) and the rear sensor (post-cat). The front one should vary quite a bit and the rear one much less so. If you only have a voltmeter, you might be able to check the voltage while the engine is running. It should go from about 0.1V (lean) to about 0.9V (rich). You can
usually force the mixture to go lean by removing a vacuum line.
You can also visually check the O2 sensors. If either one has got dirt or oil on it, it's probably due for replacement anyway. I've read some that say that they should be replaced every 100K miles anyway, but I'm not sure if that's really necessary on a well-maintained vehicle.
If you don't have a 'scope and you're willing to take a (possibly wrong) guess, I'd start with the post-cat O2 sensor.