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The Beachball rim was obtainable in Season 16: "Summer Beach Party" by being in the top 2% when the season ended.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Beachball's current MU count is at ~222, which is quite high compared to most other rare items (such as Champion Trumpets or Sled Void), however its still incredibly difficult to spot in trading, way harder than majority of the uncommon items.