There are several kinds of "earthworms" and the one you want, as a killi-keeper is the "red wiggler" that is the smallest you can get; It gets about three inches long at most. Any good bait store should have them.
To culture them, prepare a box, a Styrofoam shipping crate works well, by placing a mixture of 2/3 peat moss and 1/3 well composted manure. Add about three portions of red worms for the bait store, and feed corn mean every three of four days.
In about a month you should have a lot of worms in various sizes. These worms are fairly tolerant to temperature - don't let them freeze and don't let them bake. They should be moist but not wet. Harvest them by picking them out with your fingers.
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