In our country store, the all the school kids make up ads and post their names and numbers up on it for "summer jobs wanted" When someone needs help, they just call down to the store and get some numbers. You can also call your local extention office and notify them that you are willing to do stable help etc. if they know of anyone. Even if you don't drive yet, many people would be glad to pick you up. I know I would!
When you live rural, don't kid yourself, plenty of people in the neighborhood could use a hand here and there. You could offer to babysit, clean house, garden, anything to help bring in some horse money. Then budget that money carefully so you will still have some when school goes back in session.
You could also ask your parent's to give you an allowance each week off the top on payday so you could budget your horse money and take care of expenses that way. I'm very good at budgeting, so if you need help, just pm me anytime.