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The Rainboom Drift is obtainable from the Tier 5 Safe (costing 100k per safe) with a 28% chance of dropping.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Rainboom, being the newest drift from the Tier 5 safe, being of epic rarity with only a 28% chance of dropping, makes it not so cheap to get, if you want to gamble for it. It can easily be exchanged for other 200K items due to this, such as Ray 9, Stallion or occasionally a 250K limited (like a radiant).