In one our former homes we had that field stone/ boulders around our fire place, we didn't like it, it was too bulky looking for the space.
These were faux boulder with a flat back and super light.
Went to Northland construction and picked up some ledge stone which was supper easy to install and not that expensive either.
http://www.northlandconstruction.com/
You can try Habitat for humanity re-store as well see if they have anything there.