How do you get collector accessories?

The practical reading of this is that accessories are a reward for breadth. What fills the Animopedia is catching different Aniimo, not grinding the same one, so the collection you end up wearing is a record of how many species you have actually put in the book. Play the catching loop as a survey rather than a farm and the accessories follow on their own.

That also makes them free in the sense that matters. You do not buy them and you do not roll for them. The cost is time spent finding and capturing Aniimo you have not caught before, which is work you were going to do anyway if you care about the roster.

The accessories you cannot catch your way to

Not every exclusive accessory comes out of the Animopedia. The console pre-order editions carry their own. The $20 Collector's Edition pack bundles exclusive in-game accessories with the Collector's Companion Guide, and the Advanced Edition pre-order bundles exclusive accessories with the Advanced Companion Handbook, which is the standard battle pass tier.

There is also a cosmetic reward outside the pre-order packs entirely. Hitting 30 million pre-registrations unlocks the Sunlit Myriad limited outfit set, so that one costs nothing and depends on how many other people sign up rather than on anything you do in game.

Not settled: Which Animopedia entries pay out which accessories has not been laid out, so there is no way yet to target a specific piece by hunting a specific Aniimo.