7th Inning Balloon Tradition Questions

Coming to Japan in September and will be going to a Lions game at the Belluna Dome (picked them because Saitama is my home town's sister city here in Canada).

I want to know more about the 7th Inning Balloon Release tradition I'm seeing on clips and broadcasts as I watch more games. Do fans get handed the balloons, or do they buy them at the game, or are they bringing them to the game? My kids will love being a part of this (and I'm looking forward to it just as much too)! Anything else interesting to know about the tradition? Thanks.