NOTE: This solution involves deleting your global saves. I don't know if it works with stars, but there are more than 35 stars in the game. Don't do this until you've gotten all of your stars.
Skins can be found in treasure chests around the world. Unfortunately, going back to chests you've already been to only gives you coins. I was stuck at 97% and couldn't find any skins.
I deleted all my saves and started going through the tutorial. I found the first two skins (Nigiri and Maki) in the tutorial (one in each world). But it didn't pop. And the tracker didn't go up.
As I sat here typing up the beginning of a solution, the achievement popped in the menu. Mind you, it was a VERY late pop, but it popped. My solution, therefore, would be that if you are down to a few (and until someone compiles an actual skin location guide), go ahead and delete your global saves, run through the tutorial, and wait. It took maybe 4-5 minutes of me sitting here ticked off, but since it worked, I'm passing the information along.
Hope it helps. Best of luck!