Where do i buy a CA approved Catalytic Converter??

I’m going on a roadtrip soon and need to buy a catalytic converter to replace the one I have.

For some background, when I got my car the guy replaced the cat (not sure if it was the original even before he changed it) since it was failing. A couple months later it started failing again. Some months ago it stalled on the freeway (it got clogger). Got it cleaned out and it’s working but check engine is still on and I’m scared of doing a 7 hour drive and possibly stalling again. I wanted to get it from the dealer but don’t have $1800 to spend, especially since the trip alone is gonna cost me money. I need a place to get a good cat or not sure if I should just get a cheaper one and save money for the dealer one after I come back. I’d really appreciate any suggestions.