Depends on if you need a carb number or not.
I get cats from eBay. They list them per engine displacement and in/out size.
I've. Ever had to deal with a picky visual inspection or a fussy station looking for carb numbers for anything aftermarket though.
You'll pay more for a cat with a carb number, and very little for a high flow bullet cat.
Do some searching and ask questions before purchase if you need to.

 

There's several companys that sell them that are good quality like Cherry Bomb, Flowmaster, Magnaflow, etc. When it comes to catalytic converters or mufflers, don't buy anything from a company you've never heard of.

As far as cost, you can spend anywhere from 75-400 bucks for one

 

On my 1990 460, Im running all stock, into a stock cat then a 3" pipe into a flowmaster 10 series, and then dual 2.5" out. This is their loudest exhaust system available, but the stock cat is making it much quieter. I was going to buy a Hi-flow cat from magnaflow (ive heard good things abou them) then I found out I dont need air care, so im going to delete the cat completely. Hoping for a very big increase in sound and tone. The cat is more restricting than the muffler itself in most cases.