You can now get green lacewings and trichogramma wasps--perhaps the two most beneficial insects--at nurseries. Lacewings eat aphids, mealybugs, scale and mites; the tiny wasps parasitize caterpillars, including tomato hornworm and cutworms.
Actually, what you purchase at the nursery is an empty shipping pouch, which you then mail to the insectary. They fill it and send it back. The cost of the pouch includes postage. If you cannot find these packages at a local nursery, telephone Orcon, (213) 937-7444, for a nursery near you that carries these products.