As with many things, the hardware store is probably going to be rather expensive and transport is a hassle (most people do not have a car with enough space to transport solar panels). So you order this stuff from specialized companies who will deliver it to your doorstep in one more or less convenient shipment. Same for pretty much any bigger, more expensive things needed when building or extending a house. Hardware stores are convenient if you quickly need something but that's mostly it, IMO.