If you properly take care of your lawn mower, it could potentially serve you for a decade or more. Your lawn mower's battery won't last you nearly as long. With regular use, your lawn mower's battery will only be around for around 50 hours of use before you have to replace it. But finding that replacement can sometimes be a hassle, especially if your lawn mower is getting on in age.
Contact the manufacturer. This is the safest and most secure (maybe also the most expensive) place to get your lawn mower battery. Call the manufacturer with the make and model number of your lawn mower, and if the machine is still in production, they will be able to send you a new battery.
Contact the dealer. If you bought your lawn mower from a third party vendor, or there's one in your area, ask them if they sell batteries for your particular lawn mower. Have the make and model number on hand before you call.
Find a website like Batteriesplus.com that specializes in large batteries. But be careful when you shop. Certain unscrupulous websites have been known to sell fake batteries. These can usually be identified by their unbelievably cheap price tags. If it seems too good to be true, it probably isn't.
Contact a lawn and garden store. Sears, Home Depot and other home improvement stores often carry batteries for popular brands of lawn mower. Call or visit them online where you can search for batteries by manufacturer.