most municipals have waste management processes, many areas have mines, and or industrial zones which you might raid for material like rock dust to enhance minerals. These companies, typically here at least, must be able to say whats in the material, so you can have a look and see if it suited. People will often drop green waste and woodchips to you if you have space to take it. It is possible with some work, to source everything you need for free, where not you might read books like "The ABC of Organic Agriculture - phosphites and stone meal - Jairo Rivera - Julius Hensel
in this there is a section dedicated to the worlds peasant farmers, with many ideas on how to circumvent costs or at least lower them