I have a 02 Z28 and the cats are going out. Where can I buy direct fit cats or can I use any universal and have it modified to fit? They have to pass smog in Cali.

 

I replaced mine with a Random Technology cated Y-pipe and passed with it in September.
Its not smog legal but I did pass smog with it here.
Another option is go to the GM dealership and order new cats, or if you have someone who can weld them up them you can use some universal cats, but that not smog legal either although it's probably the least expensive.

 

Does the cat warranty apply to modified exhausts? I have Edelbrock shorties with a TD setup.

 

that would be up to the dealership but i would try and ake them do it and if that idnt work i would put the stock headers back on anything past the cat shouldnt mater