Where to find gonunaba?

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[Migel]

It's a Legendary weapon. Just finding it around is not possible, as I think they removed Gold Weapons from quarantine crates.
In order to get it, you want to become Underdog rank in Invasions, then you have a 50% chance after every win to get a golden weapon. It might be one of them. Alternatively, you can look around wether people want to trade their gonunabas for any items of yours.

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thank you

[Blade Master]

1/1,000 chance to find it on Hard mode, 1/100 on Nightmare mode from looting.

But the best way to earn it would be invasions, Migel explained it already. You need to be Underdog or above, however you have 50% chance of obtaining any fantasy Gold-tier weapon after each victory and defeat, it doesn't specifically have to be win, just to make it clear.