The soil that you buy in the store usually contain some of the benefical microbes from the composting process but you can also buy certified soil that should contain most if not all.
One of these microbes is the streptomyces actinobacteria that grows in compost. It kills mold and other harmful organisms and promotes health in plants by reinforcing other microbal processes in the roots, including the mycorrhiza fungi. Mycorrhizal fungi on the other hand is a mutualistic root fungus that exchanges nutrients with the plant.
You can buy dormant spores of these organisms from advanced grow and gardening shops. Usually you have to order them separately becuase it's not that common that people ask for them. The easiest way to get ahold of it is to order online. Do a search on it and try something like "plant supplement + mycorrhiza + order".
is one place that you can order it if you live in the U.S. They will also ship overseas but you can most likely find some closer to where you live. The product is called MycoMaxx and should cover your needs. The spores will have to be stored in the fridge (not the freezer, that will probably kill them) in order to remain alive and dormant until you add them to the soil. You can either plant them at the same time as you pot the plants or just add the spores to some water and pour it in. I recommend that you plant them in the middle of the pot or all over so that they will quickly find the plant roots. These microbes will also feed on the nutrients in the soil, so make sure that you supply enough for them and the plants as well.
If you know where to get fresh compost, even better. Fresh compost will most likely contain all of these microbes as it is usually allowed to mature in an outside environments but sometimes also in large containers.
Outdoor soil also contain mycorrhiza and other microbes, most probably it's already growing in your garden if you have one. The risc of bringing in pests is big however so I recommend you order the stuff instead.