You can get a battery at Wamart, Sam's Club or most any place that sells watches. You would just take the watch to the store and they will put a new battery in for you.
I have so many watches I would go broke buying batteries from the store --I put "watch batteries" in the search and got several sheet of them --about 25 batteries for about $20. better than $5-$6 per battery-- You have to know what size battery--marked by a number on the back --after opening several of my watches -- I saw they all took 376/377--easy to replace- pry the back off with a knife; snap it in and put the back back on--tap it down tight. Also a tip; to make your battery last a lot longer --pull the stem out when not using-- just remember to set it again! LOL... (Walmart used to put it in for you but won't anymore)