Hello SuperPiggy,
If they are broken there are no formal places to buy them. If they are just loose and you have an Apple store or authorised re-seller in the vicinity, take the mouse in and they may do it there are tell you which glue you can use.
Some workshops used by authorised re-sellers may have kept damaged mice as spares so they may help too.
If your profile is up to date, you are using Mavericks. Until you have upgraded to El Capitan, don't be tempted to buy a new magic mouse 2 as it will not function properly with Mavericks.