Steph, check equiery.com. They have a section ("Selling and buying" -> General) where barns are looking for help. I'd also suggest to contact hunter / jumper barns in area directly and ask if it's something possible. I'm not into jumping much (so can't suggest any barn), but I know dressage barns look for the help in exchange for lessons all the time. Good luck!
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."