Try the £1 shops. You can usually get a card of batteries with about 6 of each and eight different sizes, approx. 48 in total. Last time I got a card I 'repaired' five watches in the first week. Even if they aren't as good as the branded items at around 2p each they are certainly a bargain. Hardest bit is usually getting the back off the watch so that you can do the job.