Check out the Solos for a Young Violinist books by Barbara Barber.

I doubt you can find anything worth playing for free, its best to buy the music especially for contest anyway.

watch out. for contests you sometimes have to have the original on stage with you on your stand and copies of this given to the judges. i know thats how it works for chamber music comps anyway. have you got a piano accompanist? and what grade are you about. (i haven't sat any, but my teacher thinks grade 6 (english grades like trinity))
Mozarts pieces are fun. but sometimes can be more difficult then they appear. ask your teacher. they usually ahve A LOT of music and may be able to accompany you. if you dont have a teacher - GET A TEACHER they are very helpful in preparing you for comps.

if its a school comp it might not be so strict on rules...