Call the local stables and tell them that your situation. Tell them that you can't afford to pay money for lessons, but if they are willing you could pay with time (mucking stalls, grooming, longeing, etc.). Keep in mind though, some of the places who would let you work off lessons will want you to pay for the first few. Or come in without the time going towards a lesson, to learn about basic horsemanship and the specific jobs they will have you doing.
I can't afford lessons at the moment either. So I'm out at the barn once or twice a week doing odd jobs around the barn in exchange for lessons (and usually my trainer will also let me hop on for a little ride/not lesson on the days that I work too