If stone, they are most likely granite but chances are they are tumbled concrete 4"x4"x4" pavers. Similar look but at half to a third of the cost of cut stone

Have you checked local masonry supply or stone yards? My local Home Depot sells the granite cobbles by the pallet (250 pieces). and the pavers are carried by most masonry supply yards.

Check Belgard Pavers, Cobblestone, in Pewter color. They make a square shaped paving stone that looks like granite cobblestones.