Your choices are as follows:
1. Contact the companies direct. Some will sell them to the public but will charge, others won't. (what company are you looking for?).
2. Try e-bay or similar, however, there is no guarantee as to the quality.
3. Contact a printing firm, although be warned that stickers are copyrighted so this option is only really if you want your own design.
I would also warn that stickering a bat takes practice to get it right, you need a good eye and steady hand. If you attach the sticker wrongly it's hard to remove and may end up lifting the wood.