The materials for new clothes are stored in unique chests (i.e. chests that have materials that are in limited numbers). The best way to find them is to upgrade your treasure finder using a gold nugget. The treasure finder will charge as you explore an area and your chances of finding unique chests will increase.
You could try just leaving it to Nel but the best option is to explore yourself and keep chugging energy potions so that you're always exploring with the treasure finder 100% charged instead of having it reset whenever Nel goes to bed.
As for the dancer outfit, those items are in a secret area. Assuming that you've reached the end of the latest update's story then the elven lady gave you the key to unlock it. You just have to figure out where to use it.
Look at the key's design and think about what areas you've previously visited that matches with it.
If you get frustrated then you can just scroll though the comments section and you'll probably find the solution. That or you can just ask me.