Hey Katy! You can find them at any supermarket.
For bread, look for harina 000, which is a refined wheat flour with gluten. I usually go for Morixe or Cañuelas.
For baking, you'll need harina 0000, which is even more refined than 000. Same brands work for this too.
You can also check out "dietéticas", which are small stores that sell ingredients by weight, including different types of flour. They often have a good selection!