You would source them locally in person. A landscape supply is the best place to go. You should find a few different colors of flagstone depending on where they are from, i.e. WI, CO, TN. Also matching (or contrasting if desired) drywall and steps. Big box will sometimes have a couple pallets to sell in spring, but the landscape supplier will have a constant source and the veining of the rock will be consistent for that truckload.