Besides compost, what are some good garden soil additives and where can I get them?

Firstly how do you know what additives your soil needs if any? You have to test your soil ph, then if changes are required see your local nursery for the additives you require, It depends a lot on what you are going to grow in these areas as to what you may need to add. All plants have there own specific soil requirements. Maybe you have very clay soil? very sandy soil, all additives will be extremely different in each cirstance and requirements of what you want to plant or have planted can be the same,