You could buy online, or you could shop at a physical auto parts store.  Since shipping a battery can be prohibitively expensive, your best bet is to buy a battery at a physical store.  Doing so will also make it easier to make a return, if necessary.  Plus, buying a battery at an store often includes free installation. Compare the price of car batteries sold at different shops.  If possible, browse prices online or call the shop to find out how much they charge for the type of batteries you need.  Doing so will save you time and effort. Before leaving the auto store, be sure that the battery you’ve purchased is the right one for your car.  Auto shop employees should be able to look up the make and model of your car to determine if you’re buying the right battery.
Decide how to buy. Shop around. Confirm you’ve got the right part.