Start slow, ask your teacher for some guidance (her favourite sites, what to watch out for, and so on). Check out my links–I don’t promote or sell crap.
Buyer beware! ┬áMany items for sale may not be made for bellydance, but are marketed as such. Usually, you get what you pay for. I’ve seen a lot of items purchased fall apart or tarnish or simply have the wrong look, culturally. Be careful when it comes to parting with your money. ┬áBe aware of shipping rates as well.

If you see 42 copies of a DVD on eBay, chances are it’s not very good.

Ask or check into where the product is made–there are a lot of goods coming out of India, Pakistan, and China, but outside of some lovely fabrics, most of the jewellery, belts, and beading are not meant to put up with the rigors of bellydancing. The thread is often too thin, not sewn well, or sensitive to friction.
Consider the recent health scares with lead in toys and cosmetics–there is also a big concern with low-end metals used for costume jewellery from central and east Asia. Protect your health.
Be aware, also, of the style of costume suitable for the style of dancing you are doing. Cholis and tassels and bitty bells are not suited for Egyptian raks sharqi, and filmy see-through pantaloons flatter no one but Barbara Eden on “I Dream of Jeannie”.
I prefer to buy items from Turkey, Egypt, or North America. I shop from 1 or 2 sites, usually Dahlal Internationale, at <www.dahlal.com> or Horus & Isis.