How do I know what catalytic converter to buy?

Last month I took my car to AutoZone to do a test to see why my check engine light kept coming on and going off. They gave me a piece of paper saying I had to replace the catalytic converter. I lost the papers they gave me and my check engine like it currently off. But my temperature warning light comes and goes as well. (Which I’ve never had any issues with my car since I got it in 2018) When I go online to buy one, I keep seeing all these different catalytic converters that fits my car, but I want to make sure I’m buying the right one. I’m trying to avoid going to a mechanic because I know they’re going to charge me a arm and a leg to find one and replace it.