The watch uses a size 1632 and the strap uses a different size but I can't remember the number. I got the battery at my local ACE Hardware but you can purchase both sizes at any big box like Target or Wal-Mart
My hubby changed mine out...just popped off the back and gave me the old one so I could take it to the store and get the same thing. It's one of those flat camera-type batteries. I had him change out the one in the sensor at the same time. As Unident said, any watch or jewelry store would be able to do it too.