I've looked for horse jobs for a few years now, they mostly all want someone 18 or older, BUT almost any barn you go to will allow you to work off lessons/leases/board. The one time I asked we got to do minimal work- muck out half the barn, fill the water buckets for all of it- and my cousins and I all got $10ish off our lessons. As long as you get what is WORTH your work I highly encourage it. That way you can lease and take a lesson maybe every other week.
As for finding leases I go to:
Equinenow.com type in your area and then look for any title that has "lease" in it
Horseleaseconnection.com. You have to get an account but otherwise it's a good site.
And of course, craigslist!
Or, when you find a good barn, often they'll let you lease even if they don't really "advertise" it.
This year is actually the first I'll get to lease...I'm so excited!