I get it from local landscaping firms and/or greenhouses.
HOWEVER, we live in Pennsylvania - the mushroom capital of the USA.
We don't live near the mushroom farming area in southern PA, but I understand that mushroom farmers will fill a large truck with the spent soil, and sell it to landscaping firms, etc. throughout the area.
I doubt if they go to Arizona, though!
If I couldn't get this and needed soil amendments, I'd probably buy top soil, or compost, or composted manure - whatever I could find locally. Compost, by preference.
Pat