Craigslist - should be one for Harrisburg or at least Baltimore. Probably some people offering free manure in the Free section or Farm & Garden. Also see if there is a Freecycle group (see freecycle.org).

Municipal yard waste dump/mulch site? Have you checked with the city?

What happens to yard waste in your area? Our city trash service hauls yard waste to the landfill where it's shredded, composted and screened. I can get a cubic yard for $12, usually they give me way more than that much on my trailer.

Tree trimming services including the ones owned or contracted by utilities. My rural elec. co op has a yard where they dump wood chips and I can load all I want for free. Some utility or tree trimming/removal services will dump them in your driveway just to get rid of them.