You can buy typical scarfs that can be used as hijabs for a very low price at most department stores, like Zellers, Walmart etc. Second hand stores are also likely to have them. If money is a problem offer to do some task (ie lawn cutting, chores around house etc) for a small sum of money. I bought several large scarfs for $10 a piece which work great. Dollar stores also have scarves but these tend to be smaller but for a dollar, they work especially if you are young and have a smaller head than I do. You can always buy two of the same pattern and stitch them together.
At one time http://www.youtube.com/user/iloveelh...ature=g-user-u had a contest for a free prayer outfit at one time. No idea if they will have one again in the future.
Also keep in mind that scarves in general are a fashion accessory that by themselves don't mean Muslim. You can just ask your folks to buy you some to wear around your neck at school or over your hair to keep it dry or from being blown about. Then you can just wrap them to not only cover your hair but neck and bosom as well.