If you are buying I would get a truckload or however many you need of good a compost mix such as mushroom or manure compost, just make sure the manure is composted well. For soil buy a good topsoil not all topsoil is good, check where they get it from. Then start composting yourself with leaves etc. to add later. I love my raised beds, just wish I had made them higher, so I could sit on the edges and work without bending over. Make sure to check your PH and if you have a good compost and top soil you should be ok for other nutrients.