Bolt on cats will usually be expensive, such as $150-$300 each. Universal cats are very easy to buy for about $50 each. Any muffler shop has the skill to replace a cat like that, but a great number will not do it.
Your old cats need to be bad before you set about to replace them. If you want to consider saving that large amount of money, call the shops where you live and ask them if they will replace a cat with a universal model. Many shops will stock those kind of cats, if they will do it you can have it done in one stop. Good luck,