How can I purchase a battery for my airtag

See the following Apple Support article for information on Apple AirTag batteries: How to replace the battery in your AirTag - Apple Support

including the following:

Insert a new CR2032 lithium 3V coin battery (available at most electronic and drug stores) with the positive side facing up. You'll hear a sound indicating that the battery is connected.

CR2032 batteries with bitterant coatings might not work with AirTag or other battery-powered products, depending on the alignment of the coating in relation to the battery contacts.