Autozone, checkers, pep boys, any auto parts store has catalytic converters.
They will either have one made for your car or have a universal cat.
Most stores will beat other stores prices if you can show them its the same part.

 

I got it from a shop around town, can't remember the name.

 

If not manufactured specifically for your model, watch the height of the cat.

 

i doubt you need a cat. my sisters and g.f car recently threw a P0420 and i changed the o2 sensor in both of them and they both said the car runs alot better. the shop i brought it to also said i needed a cat but if i was you i would just go to autozone and get a o2 sensor and replace the pre cat (befor the cat) o2 sensor. also that code appears when you have a leak in your exhaust so check your exhaust for leaks. if their isnt any then i would just swap the o2 sensor