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The Arch9000 was obtainable in Season 28: "Cyberpunk" by being in the top 2% when the season ended.
Arch9000, whilst holding a relatively low MU count of around ~180, isn't necessarily a hard tire to spot in trading. With it having around ~32.5K copies, it's far from being a rare item. Another uncommon tire, the Kanagawa has pretty similar copy and MU count to Arch9000 (30K copies and ~160 MU), despite currently being listed below it in rarity.
Supply is widely distributed — no single holder owns enough to register as a hoarder.