does anyone know of where i can find a cat with a 2.5" inlet with a 3" oulet. is it possible to get one with those dimensions or do you need an adapter piece to make it work?

*Car sold*

It would probably be MUCH saner to get a high-flow catcon from someplace like RRE, and weld an adaptor onto it. I think that if you could find a catcon like you mentioned, it would cost more than a 2.5" or 3" catcon, an adaptor, and having somebody weld them together.