I'm expecting my first in a couple weeks, so I haven't had experience with all of this yet... But I'm sure that you could find the Mother's Milk tea, or something like it, at a health food store or maybe even a pharmacy/grocery store. I found a brand called Yogi tea "Nursing Mother," at a health store here in Indy.
Things I've heard that help with your milk supply is drinking TONS of water (you're supposed to get something like 86 oz a day as a nursing mom vs the regular 64 oz). There are also supplements of fenugreek, blessed thistle, and alfafa that are supposed to help. Also, my doctor told me not to use decongestants, because they can dry up your milk supply.
Have you tried pumping at all? The body is supposed to respond to the demand of the child, so the more milk that gets taken out of your body, the more your body will produce. It could be that you're worsening the problem by substituting formula. (Although, I know you want your baby to be getting enough food.) You might try pumping milk every few hours, to empty your breasts as much as you can. That should signal to your body to make more milk.