Most grocery stores, banks and drug stores sell postage stamps. However, special stamps or different values of stamps may only be offered at post offices.
The USPS website offers a locator tool where the zip code can be entered, and a comprehensive list will show all the stores and post offices in that location that sell postage stamps. There is also information on what type of stamps are available to buy like single stamps or booklets of stamps only.
Buying stamps to mail letters and packages from post offices helps to keep them open. With the reduction of letters being mailed, small post office locations are having their hours shortened or are being closed.