Where is the costume and swim store?
The town map will show you were the store is located so you can build it
The picture below shows how the store looks when its been built:
How do you use the store?
When you click on the store you will see the page below, it will show all of the sim life stages, there you can select the clothes, hair and makeup you want to buy for each stage.
The first tab is wardrobe, there you can find all the clothes and hair you already have available.
You can only buy 2 outfits for babies
Click on the item you want to buy, it will then no longer appear in your store, it will move to the wardrobe and can be worn by as many sims as you want in that life stage
Once you have built the store you will be able to find these clothes in your wardrobe: