Contact all the quarries and masonry supply yards within a 10- or 20-mile radius of your home. While these places prefer to sell their rocks, they might discard small or irregularly shaped pieces of flagstone. Ask if you can collect their discarded flagstone.
Talk to your neighbors and friends. Find out if someone is renovating their landscape and will be getting rid of their already installed flagstone. If not, find out if anyone has leftover flagstone from a previous project. Ask your friends and neighbors to talk to their friends and neighbors.
Contact home improvement stores and ask if they're willing to make a charitable donation. This option is most likely effective if your need for flagstone is not personal but for a communal cause, such as for a school or playground. Home improvement stores might be willing to provide you with such supplies if you show them the appropriate paperwork and proof.