Several places where I have readily found what I needed:
Lanscaping/Nursery/Garden centers...they often carry rock and stone for borders, walkways, drainage, water features and patios.
Brick Suppliers...they often carry not only brick but concrete block and many types of stone and rock.
Home Improvement stores that have garden centers, like Lowes, Home Depot, Builders Square..etc.
In some areas, even gravel suppliers may have or can get stone for drainage projects and road construction. They often have it sorted by size and it's quick to get a few in the trunk and pass over the scales.