Garden supply catalogs offer many species, and you can purchase them directly from private insectaries.
Predators are usually generalists and feed on a number of different pests.
Parasitoids are more specific and many can only develop in only one host species. Private consultants also provide identification services. Calling ahead to ask when to bring in your specimen and how to preserve it will save time. Buy organisms only from reputable insectaries or retailers that guarantee their products. This should cover facts about the life cycle of the creature, specific release rates, how often they should be released, and any special requirements of the organism.