JCWhitney has some universal Superior Performance CMX High-flow Catalytic Converters for $140:
http://www.jcwhitney.com/autoparts/P...p-8796/c-10101
They also have cheap universal and direct fit cats, though you are not going to gain a lot of power just from changing out the cat and leaving everything else stock. Unless of course you need to change the cat because it has gone bad or something.
You will be hard pressed to find good headers for cheap. I'd recomend looking into other power boosting options if you are on a tight budget