via powdered supplement (for example, repashy calcium plus or reptivite once a week or once every two weeks)
via vitamin A oil (just a tiny smidge under the wings of a cricket or roach periodically - I personally do not prefer this method)
via gutload (for example, the occassional inclusion of boiled egg in what you feed your roaches or crickets or terrestrial isopods. I personally think this is the most natural method)
via the type of prey offered (those which have this naturally in quantity)
via oral solution (a vet can make a dose based on the chams weight)