I'm not sure there is a really good substitute for breast milk.. anything that isn't that wasn't made to be digested by their little stomachs.. maybe look around your area and see if there are any mom's with a surplus stash of breast milk. I see mom's in my area always donating milk to people who need it

Similac advance is the most commonly used especially among the Healthcare providers and in the hospital. Enfamil is another good one

Not sure. We weaned to Gerber gentle for my oldest. He barely seemed to notice the transition!