Catalytic Converter O2 CENSOR?

I have a 1987 325IS BMW. It wont p smog, Every mechanic tells me after seeing the failed smog report that it's forsure those 2 parts. I need to know where i can buy a California Legal Catalytic Converter And a good o2 censor. I dont wanna spend TO much money but a few hundred is okay. Any mechanics on here know if it could be other things? I failed the HC test by just a few but i really failed the NO test by like a thousand+. I ped the CO2% test. How much should this job cost to buy and fix the Catalytic Converter and the o2 censor