I saw a few people were looking for summer jobs or temporary positions, so I figured I'd help anyone that is looking. Be aware that most of these require that you are at least 18. This is the site I used. It gives almost every major camp in the US and other countries
Horse Camps | Equestrian Camps | Horseback Riding Summer Camps
I'll share my experience:
Last year my summer was free and I was feeling adventurous so I decided to find something to do that I would enjoy. I landed a position as a wrangler at a camp in San Antonio, TX that was situated on over 3000 acres. I stayed there for 2 months. Amazing place! A few celebrities even sent their children to this camp (can't say who, we're supposed to keep it confidential)
There were eight wranglers and we had 50 horses to ride that hadn't been touched since the previous year. We arrived a few weeks before camp started to get them ready for the kids. Best experience of my life! We lived in a beautiful apartment above the barn and we could see the horses out the windows. A little exhausting but worth it. Our days started at 7am and ended at 9pm. The pay wasn't too great considering the amount of work but most of us just did it for the experience.