This worries me since I had to replace the cat on my car a few years ago. I got it online. There were no serial numbers or paperwork. I hope I dont get hassled at my next smog check.
I don't think you will. First, the smog check stations are just checking to see if the cat is there, it's the repair shops that have to ensure the new ones they replaced have date codes after Jan 01. It's when you have to replace the one you just got that you will not be able to buy one online.
You should see one of the latest idiotic things the CA ARB is considering . . . banning black cars. Due to all the heat dark colored cars absorb in the sun, black cars are a "major" source of global warming, didn't you know? Actually they claim they are not banning black cars, they just want new paint formulas by 2015 that won't absorb as much heat. Think of how much this will add to the cost of cars? And last time I flew over LA I remember most of the ground being covered in asphalt which I'm sure causes far more "global warming" then black cars.
Maybe they'll give me a green credit since the Rex-O-Saurus is white?