Before you buy a solo you need to figure out what grade you play at. Band music is subdivided by its difficulty level into grades 1-7.
http://www.hickeys.com/pages/grading.htm is written about orchestra grading, but you can get the gist of how band grading works by reading it.
How to get a solo:
1. Buy a book from a local store. Make sure that if you`re required to have accompaniment (piano or whatever) the book transcribes that part as well.
2. Buy from a double reed specialist online. www.oboeworks.com, my local shop, sells plenty of books. You can call them toll-free at 1-877-OBOE-PRO if you don`t know what to buy. They`re really nice about helping young players.
3. You might not need a solo book if you`ve got a method book with a long etude in it. If it`s just for an audition and you won`t be needing accompaniment, that`ll do just fine.
4. See if you can get hold of your school band director and borrow something from the school music library.
5. Ask your private teacher to borrow something. They probably have a million things lying around. And if you don`t have a private teacher, get one. Immediately.