What model year of the Catalytic converter should I buy for my car?

Strictly speaking you need the 2008 exhaust system to be 100% smog legal. Some states are tougher than others when it comes to enforcement. If the engine runs correctly the way you have it set up I would leave it alone.

This is kind of like asking "who's buried in Grant's tomb"? ---- LOL. Absolutely, you should use the precise part and model for your car's year. FOR is correct, it has to be 100 percent smog legal. Why would you ask which part, it's obvious. I suggest you take the car to a professional and have that done correctly; you do NOT want to mess around with this. You can get into a lot of trouble not using the correct emissions controls on your vehicle.

Thank you to you all for your answers. I'm trying to save some money and at the same time to get more exposed to working on my car. Sure, I want my car to be legal in California. The engine was replaced 4 months ago, and the car has run fine until last month when I took it for a smog check. The catalyst converter was not ready. I did drive patterns many times, but the cat. was still not ready. OBDII showed the A/F Sensor 1 at 3.2V, O2 Sensor 2 fluctuating from 0.1 - 0.8 V. No other DTC except P0420 pending at one time but never registering as an actual DTC. I think the cat. is bad, and I'd like to replace it myself. I've been looking at the Toyota Solara 2005 and 2008 cat. integrated with the exhaust manifold. It's not obvious to me since this is the first time I'm dealing with the cat. problem (I'm still learning about car), and I want to save a trip to the dealer to exchange for a correct the cat.

If this has to pass California emissions the car will be sent to a state smog referee. To do a legal conversion like this the entire emissions system along with dash lights has to be moved over to the car. Smog techs here are TERRIFIED of anything out of place in a car. Their jobs and probably their freedom is at stake.