If you are talking about Magic the Gathering, then you can buy them at most TCG online retailers. A lot of general stores like Walmart have them as well, which is also convenient. Finally, many people on Ebay or Amazon also have old collections that they want to get rid of, so they will sell you bulk cards for cheap.
There is a lot of places you can go to,Log In orLog In are some of the more respected examples. As for eBay, you can buy some expensive cards there in foreign versions, which is nice way to save money (some cards can be up to 50% cheaper and still tournament legal).