Often models don't have much clothing beside what they use in their daily lives. Especially when you're not working with professional models.
Of course you should always ask a model if they have any clothing that suits the theme you're working on. Use a moodboard to illustrate what kind of clothing you're looking for. Perhaps the model has suitable clothing or he or she can borrow it from friends.
Often, however, they don't have appropriate clothing. That is why I have my own wardrobe in the studio. That wardrobe needed to be filled so I was glad to find the site Unitedwardrobe. Here you can buy second-hand clothing directly from the previous owner.
An easy and affordable way to buy clothing. I don't think the site is live in every country yet, but it's worth a look. Of course you can always try eBay or Aliexpress too.
Perhaps you have an even better way to get great clothing for affordable prices. Let us know in the comments!