A wardrobe of garments set aside specifically for studio work is invaluable. Models always bring their own apparel, of course, but it is great to have more to choose from. Sometime, dresses and the like that are great for photography are too sexy or too colorful for everyday use, and therefore do not sell well in stores. As a result, such items can often be picked up at bargain prices. As an aside, I live in the Geylang area of Singapore, which is the official "red light" district. Prostitution is legal and regulated here. As a possible result, you never hear of any rapes! I digress though. Just up the street is a mall which has lots of amazing dresses - most likely targeted at ladies of the evening. I have purchased several from there. One doesn't see dresses like these for sale on other parts of the island.
One of my favorite garments for models is the jumpsuit. It is fantastic for slender models, but does not look good on models who are slightly heavy. Bebe has had some great designs, as has Guess and Forever 21. They work well for both waist-up and full-length portraits. Because the eye of the viewer can travel along the length of the fabric, the jumpsuit sort of "flows" and can make the model look taller and slender.
Here are a few samples from my studio wardrobe for your viewing pleasure: