If you can't offer housing, it will likely have to be someone local- unless you offer enough pay for someone to go out and rent a local apartment. I'd put flyers up at barns and tack shops in your area and post on your GMO site.
Since you won't be providing housing, I'd focus on local advertising. Does your GMO have a newsletter and/or website? That would probably be your best bet for placing an ad. You could put a flier in your local tack shop. But for finding someone local word of mouth might be best of all. Tell everyone you know that you are looking for a working student.