Breast milk is probably the best milk for the baby. Actaully it is there is no denying this! It’s just that baby formula isn’t bad either. You can do whichever one you feel like doing dear. Don’t stress over the fact that you cannot provide more breastmilk. It is completely fine and your abby will be as healthy as any other. Also mixing baby formula and breastmilk isn’t bad. I used to do it and nothing bad happened to my kid. For the baby formula suggestion a pediatrician can tell you more. They see what the baby needs and tell you more. Back than I used Aptamil most of the time. Then I used some SMA PRO baby formula too. In your case though it might be different. Whatever your baby likes and your doctors suggests. God bless you dear.