it is probably the most crucial aspect of organic growing. Beneficial microbes allow healthy absorption of nutrients/oxygen/water. just give them natural food like kelp meal and your roots will thank you.
There are many products out there to choose from, i like the fine powder myco but just go for whats cheap. I wouldn't pay more than 50$ but there are some like Great White myco which is like 100$ or something.
If you have a hydroponic store they will have it for sure but you might have to ask an employee. Since its so expensive they keep it behind the counter sometimes
Good luck!