There's the usual garlic which helps a bit but to be honest I don't think there are any suppliments that do the job really well. Similarly fly sprays help but never totally stop the horse from being bitten. I have found however that when you move your horse to a new yard they are prone to more bites & then the next season they don't seem to be bitten so much. Wether they build up an immumity or something I don't know.
I was told that Johnsons Baby Oil with lavender was a really good midge repellant...only now its called Johnsons Bedtime oil and it contains chamomile as well. A friend and I are going to try lavender oil mixed in with pig oil, which we are using any way to condition manes and tails for showing.