Wouldn't you simply inoculate more growing medium, to make more?
Is it not like any other culturation process? You take a living sample of what you wish to propagate, give it food and optimal conditions, and harvest as necessary.
So I would figure out what works best to grow what you already have, perhaps make some shelf-stable for longer-term storage in the event of an accident, and just keep making more.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
Effective microorganisms (EM) can be found just about anywhere there is bioactive soil (as opposed to dirt).
Many people promote the use of molasses (sugar) to feed these organisms and thus grow more of them.
Sugars are not the best food for any Microorganisms we want to grow in our soil, they can work but they also feed the organisms we don't want in our soil.
Better foods for growing our preferred microorganisms are complex (just like foods we eat, sugars have the same effect on humans as they have on any other organism).
So, pieces of fruit, vegetables, the leaves, stems and roots of plants, all are going to be far better foods for growing any of our preferred organisms over sugars such as molasses.
You can jump start a microorganism world simply by pouring milk on soil that you have buried corn in. Look at how Native Americans traditionally prepare soil for growing (corn uses fish under the seed with a barrier of soil between, because corn is a heavy feeder).
One of the best ways to grow EM is to use softened (partially cooked) rice that spoiling milk is poured over in a container and that then gets an addition of healthy forest soil that is nearby your garden space.
This method grows all the right organisms for the space you are going to be planting, and it is easy to do.