Well, the most important thing that you need is good energy, that will boost your learning capabilities, but if you don't know how to start and where to look for the right choice of clothes, here are some recommendations:

Children (youth and teen): hair pulled back, black leotard (any sleeve style), pink tights, and ballet slippers. 
Adults: leggings, comfortable tops, and Ballet slippers.
Men and boys: tight gym pants, t-shirt, and ballet slippers

Shoes with heels (1 to 3 inches), long skirt, and comfortable top.
Man can came with boots or shoes with some heel, comfortable pants, and t-shirt.

Tight pants and tops. Ballet slippers are welcome.

We will provide castanets and fans up to 10 students, (first come, first served basis).

Any of this clothes and accessories are available online and many online stores, such as Ebay, Amazon, and dance stores (e.g., Opening Night). The most famous brands are Capezio, Bloch, Sansha and Danskin, but there are lots of options. Also, you can find some good stuff on Craigslist, Goodwill, or other Thrift Stores. Even Target, Kohl's, Walmart, JCPenny, Macy's and Sears usually have small sections with dance apparel. 

For Flamenco shoes, there are options in reduced online stores, some of the dance stores used to offer shoes and skirts, but we really recommend you to go for artisan-made shoes from Spain. Those are probably not as cheap as others but are really durable and comfortable. We could help you choose a good pair of artisan shoes.