Before swapping the cat out, replace BOTH the upstream and downstream O2 sensors with new sensors and see if the check engine light goes away. it will cost about $100 for 2 bosch sensors from the parts place but you may just have a bad sensor. This is a DIY project you just need a O2 socket, a socket wrench, and a jack and jackstands.
And, if this does not fix it then if you buy a brand new cat to replace it, you will find the small print requires that you replace all O2 sensors when you replace the cat or the manufacturer will not warranty it, so it's not money down the rathole.
Where I live every cat that comes into a wrecker goes straight to the metal recycler, you can't find used ones that way.