@Sedan, there are many different types of mixes that you can make, and different items that you can substitute. Most of these items are a waste product from another industry - ground seed meals from food crop or cosmetics processing, bone, blood, or fish meal from meat and fish processing, animal manure, compost, wood ash, rock dust from mining and commercial gravel production, and ground oyster shells. You can use banana peels for potassium (boil them to make a tea), bloodmeal for nitrogen, and add soft rock phosphate to your soil for Phosphorus.
There should be diverse resources available, but they may be difficult to track down. Just start looking to see what is cheap and plentiful around you, and start looking up NPK and mineral content for such items - Leaves, animal manure or bedding material, corn husks, vegetable and fruit compost, used grains from making beer, used tea or coffee grounds - these garbage items can all be useful.
I started reading these forums and learning. I was disappointed in my expensive bottled fertilizers and bagged soil. I wanted to try something different. I flushed my old salt-filled soil, added castings and amendments to it, and sent it. This is run #4, and each run is a learning experience.