There's about seven million posts on cats already.
Foreign cats tend to have more of the valuable metals in them (although there are bunch of foreign ones with crappy yields)
Make sure you can identify aftermarket cats, because they apparently don't have much value in them so if you buy them don't pay hardly anything.
Domestic cats run the spectrum, from very good to mediocre.
http://www.adcrecycling.com/page/398449376
http://www.converterguys.com/converters.htm
http://www.hilltopautomotivecore.com/
http://autocorebuyer.com/prices.html
http://www.firstamericancore.com/prices.html
Study these and you will learn a lot about the different grades. I have no experience with these sorts of buyers, but they appear to be offering quite fair prices. I figure they are paying around 2/3 of the value of the total value of metals in the cat. Even if you recover all the metals (big if, particularly on the rhodium), the refiner you sell your values to will want 10% anyway - unless you have a many ounces of metals (and assuming they don't through dishonesty take more than that -you can read some horror stories here). You can start to see that you may be working thin margins trying to make money from small scale refining of cats, rather than selling them.
On the other hand if you have a good source for cats, get good extraction, and keep expenses low, then you may be able to process profitably. Or you may just want to do it for fun! It's going to be a lot of work, and you arn't going to be making big $$$ tomorrow, you'll need money, time, experience, connections, etc.
Lazersteve has a really great video on platinum metals recovery and refining, it will teach you a great deal and it's really cheap. His technique is pretty effective too.
If you are serious you will read all you can, there's a lot just here on the forum.
