Look in Hobby Farm magazine for fly wasp dealers in the add section!
Let them loose and clear your sky of every fly without chemicals and poisons!

http://www.kunafin.com/information/equestrian.pdf

The more fly's you have, the more the wasps breed, the more flies they eat until the only thing left are the wasps, then they die out, but only after they do the JOB!

It's the only way to go!