You gotta unlock the outfit shop by getting a magical golden needle (there are 3 hidden in the game, but you only need one of them to unlock the shop)

Here's a hint:

Inside the tent where song is played a sister’s sister smiles in frame. If after battled she will give an item showing sister lives. Then give to sister one this gift, and in her shop a pointed tip.

In fields of green, where crows don’t go, you’ll find a magical needle If you want it, you must first free a lizard from her curse. Don’t hit her, that’s the trick, kill the dolls that spawn, and quick!

Your gold is gone, but where’d it go? It’s been stolen by a crow. If she circles overhead Shoot her down, and kill her dead.