Some retail nurseries sell adult lady beetles and a few other beneficial insects. Mail order companies that sell lady beetles and other beneficial insects can be found through an online search.
Releasing lady beetles to control aphids on garden plants is not very effective. Most of the lady beetles will fly away in a few days. The insect that works better as a general predator in the garden is the green lacewing. Cards containing green lacewing eggs can be purchased and placed on garden plants to control aphids and a few other insect pests.
Most gardens and landscapes already have a variety of beneficial insects and other natural enemies which help control pests. You can increase the effectiveness of these biological control agents by not applying insecticides which leave a persistent toxic residue on plant foliage. If you need to use an insecticide, try insecticidal soap or horticultural oil which kill pests on contact and are less harmful to beneficial insects.
Sturdy plants can be washed periodically with a forceful spray of water to remove some pests and dust which interferes with the activity of beneficial insects. Ants also interfere with biological control and make pest populations worse. They feed on sugary secretions of aphids, whiteflies and other sap sucking pests and drive off or kill beneficial insects which help control the pests. Ants can be controlled with toxic bait which they take back to their nests. Trees can be protected from ants with a sticky barrier like Tanglefoot which should be applied on a strip of paper, tape or other protective material wrapped around the trunk.
For more information, see UC Pest Note 74140 Biological Control & Natural Enemies, http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74140.html
and UC Pest Note 7411 Ants,
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012