The answer varies depending on which stores and what locations, but many (though not all) CVS stores, as well as other drugstores like Walgreens and Duane Read stores, do sell postage stamps. Many grocery stores and some convenience stores sell stamps as well. At many Walgreens stores, cashiers usually have books of stamps for sale at the cash register; all you have to do is ask.
Some stores that do not have stamps at the counter or customer service desk often have stamp vending machines in the store. This is common in some (but not all) Wal-Mart and 7-11 stores.
Some of the stores mentioned above, however, do not sell stamps at all, so you may want to contact your local store before visiting to confirm.
Be forewarned, however, that most stores that sell stamps only sell them in books of 20, so if you only want one or two or a few stamps, you may have no choice but to the visit the counter at your local post office, as even the US Postal Service no longer has stamp vending machines in many of its locations.
You can also buy stamps at USPS website.