It is not necessary to purchase lady beetles.  They will come into your garden on their own as long as no insecticides are sprayed and they have insects to feed on. To attract them provide a diverse mix of flowering plants that provide a pollen and nectar source. For additional information on attracting and conserving beneficials in your landscape see the attached publication. It also lists plants that attract beneficial insects. See - HG 62 IPM Series: A Common Sense Approach to Managing Problems in Your Landscape http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG62%20IPM%20A%20Common%20Sense%20Approach%20to%20Managing%20Problems%20in%20Your%20Landscape.pdf

Here is a good article on the purchase of lady beetles. http://extension.colostate.edu/topic-areas/insects/lady-beetles-5-594/
mh