You could buy a rain barrel and run the downspout to it. I have found most of these barrels very expensive, hard to clean, and the outlet is not at the bottom side of the barrel. Here is one of mine that receives water directly from the spouting. It is elevated on blocks so I can get a watering can under it or attach a hose to water my blackberries. The top is covered with a window screen held with a bungee cord to prevent mosquito breeding. I think mine was free from a car wash and contained detergent. You can link tanks together so they will all fill to the same level. Install a plumbing nipple at the bottom side of the barrel, add a hose Y, then connect them all with hoses from a cloths washing machine. There is a diverter designed for downspouts. I use them on my big tanks connected to the barn downspout. I divert water to the tanks until filled, then let the water run to the ground. This is the water I use for my orchard. There are kits that connect the barrel directly to the rain barrel. Mine came from Home Depot. They work if you carefully follow the instructions. Good luck!  I hope my pictures transport OK.