When you visit a rock supply business, they will have the rocks separated by the size and type of rock. They are usually sold by the pound, price based on what kind of rocks they are.
Smaller rocks are usually separated in cages or baskets by type of rock. Larger ones might just be in a large pile.
Some places offer a way for you to carry the rocks you pick out, but not usually. This store lets you borrow a small wagon. If you have your own wagon or cart, you might want to bring it along when you go shopping for rocks to paint.
When you are done picking out rocks it will be time to weight them. Each rock and stone business may have their own way of weighing the rocks. At this business, the rocks are put on a heavy duty scale, a few at a time, separated by the rock type. The rocks on the scale in this photo are for my larger custom pet portraits. The rectangular rocks on the wagon were used to paint houses on rocks.