Breast feeding milk supply

Ch

My milk supply has always been low. I’m drinking water by the gallon and eating healthy food and snaking every few hours. What can I do to make more milk??? I’m only getting .3-.5ml every 3h of 20min pumping. I even rented the Medela Symphony pump to try and get more milk. Baby is on formula mostly because I just don’t make enough and I wish I did more! Did anyone ever find anything that upped their milk supply that I could try?