The Big Man one is already sold out.
As is tradition, the latest Amiibo announcement will accompany a Nintendo first party release.
Amiibo for Splatoon 3's Deep Cut - Frye, Shiver, and Big Man - will be available on November 17 (the same day as Super Mario RPG). The Nintendo announcement does not confirm if they will be sold separately or in one package; previous Splatoon Amiibo have been sold together.
Tapping the Amiibo will give exclusive gear as well as the ability to "snap a photo with these Splatoon 3 stars".
