You can buy mychorrhizae and some soil microbe innoculants at plant companies such as those Rick Corey lists above.
But I suspect that trying to feed a few microbes into disturbed soil or into a hydroponic set-up really misses the lessons of what microbe-plant relationships are all about.
For example, here some 33,000 microbes were identified from a sugar beet field. But what was effective against disease was the interaction and community activity of the whole microbiome of the field that developed over several years.
http://newscenter.lbl.gov/news...
It is a little like capturing a few gorillas in the wild, putting them in a cage, and concluding that these gorillas in cages represent what gorillas are like. When in fact gorillas live in highly organized communities, taking a randomized sample does not give you information relevant to the highly organized communities and territories that are relevant to gorillas themselves.
I suspect what we will learn about the microbiome relevant for gardeners, is that you can grow your own by setting up the right conditions, just as you can grow soil animals like angle worms, or make compost. Its not something you can buy in the store. It could be that the microbiome that develops in one sugar beet field, does not have the same components as those in a sugar beet field down the road.
This article suggests that you can grow your own microbes with compost tea:
http://www.gardeningwithmicrob...
