The hoover fly (syrphid fly) is attracted to flowers. The adults mostly eat pollen and nectar. Some flowers that are especially attractive include wild carrot or Queen AnneÂs lace, wild mustard, sweet alyssum, coriander, dill and other small flowered herbs. Another name for the hoover fly is flower fly or hover fly. It is the larva of the hoover fly that will feed on the larva of the white flies. They also feed on aphids, thrips and other small insects. I don't know anywhere to buy them, perhaps someone else does. I have included a link to a very helpful post on Gardenweb on controlling white flies.

Hope this helps,
Carole

Here is a link that might be useful: Controlling White Flies