I have very good success with Animed’s Buglyte, also a garlic based supplement. The horses eat it readily despite the strong smell.
I have not had luck however starting a supplement-based fly program when in the midst of fly season. I start my boys around the end of February, as ticks can come out that early in my area. From what I understand, non-insecticide supplements work best if a colony of flies is never established to begin with - fighting an existing colony is hard.
For traps, I have VERY good luck with Victor traps http://www.horse.com/item/victor-poison-free-disposable-fly-trap/BUA11/ I don’t bother with anything else any more.
You can also take steps to reduce fly breeding, clean up all manure, keep stalls dry and clean, don’t let any water stand or puddle, clean up damp uneaten hay. Manure piles should be covered if possible and compost piles should be as far away from the barn as possible. Keep manure implements in a sunny spot so the manure dries quickly and attracts less flies.
Finally, when desperate, I’ll mix some cedar shavings into the bedding. They can be found in the pet aisle, usually around the rabbit and ferret items. Be careful, some horses are allergic to cedar and will break out in hives or worse.