Mushroom compost is the "stuff" commercial growers grow the white mushrooms you see in grocery stores. As I understand it, it consists of horse manure and hay-ground up into a powder and sterilized so it has no weed seeds. You can make your own compost but it takes a lot to fill bed initially that's why I used the mushroom compost. I'm making my own compost to replenish the beds. I expect most any place that sell mulch would have it. Most in east TN do anyway. I bought mine from Newport Sand and Gravel. $42 per ton. Took about $125 to fill all my beds but you don't have to fertilize at all so that offsets the cost somewhat.