Best supplemental formula????

I've been exclusively breastfeeding for almost 3 weeks now. My baby is constantly hungry so I went to see a lactating consultant and she says that my son is younge tied and he has transferring problems. He's not even back to his birth weight yet; anyways, I scheduled an appointment with an ENT specialistic next week but in the time being, I'm breastfeeding and pumping but my baby is still hungry. Anyone supplement formula in between feelings? If so, what kind?? I want to pick up some formula to supplement.