Hi,
This is anecdotal, not at all scientific, but I have had no problems with milk supply (DS now just over 7 months old, fully bf) and I have been taking brewers yeast in powdered form (on toast) most days since pregnancy. It is a bit of an acquired taste, I've found. I couldn't say whether it has been a definite factor in sufficient milk production, but I'm sure it hasn't done any harm.
I also take a herbal tea - Traditional Medicinals Mothers Milk tea, which is nice and relaxing, even if it does have to be re-heated several times before I get to the bottom of the cup, most times!
Cheers,
Tania