Hi, I picked this off the net for you.
Microworms are harmless nematodes that live in soil just about everywhere in the world.
According to the "Encycloopedia of Live Foods," by Charles O. Masters (TFH, 1975), "It has been computed that the top six inches of an acre of soil actually contain billions of these worms."
Masters also suggests the following method for obtaining a starter culture:
"A small piece of potato or a three-inch potato with a half-inch hole bored through it is buried in a garden or wood lot for about one week and then dug up for closer inspection. Any worms feeding on the potato can be collected by washing off the potato in clean spring water and filtering the water through muslin."
(One inch = about 2.5 cm., so three inches is about 7.5 cm., and a half-inch is about 1.25 cm.)