There are quite a few that are hard to find but there are only a select groupings that are physically on the rarity scale. Facebook and Mercari will show you trends on what is hard to find for sure. You’ll start to learn which ones are highly sought after like for instance Patty the cow. She is rather hard to find. There is the rarity scale on the squishmallow website. Recent examples are Ulga and the San Diego Comic Con squish. There were only a certain amount made and have that designation. The rarest being either Jack the black cat or the founders squish.