Hey, my daughter is just 2 weeks old and drinking lots of leche (being said about her "comiendo como su pa'i"), so I am new to this...
but got some info for you:
There is a baby-formula I heck don't know the name, it is the only one sold in the country for newborns. You can ask about the "leche para recien nacidos". I never bought it in Bavaro, but it is sold in supermarkets and big colmados, maybe even in pharmacies.
IF it is sold in bavaro, your most likely bet would be the supermarket in the Cortecito or the supermarket in one of those plazas midway between Plaza Bavaro and Friusa. Don't know the name of the plaza, but there is only 1 supermarket. Costs 600 pesos for a pack of 16 8oz bottles.
If your child is not a newborn, you can buy "leche en polvo", there are many brands and formulas, Milex is the most popular, there are some that are more expensive but have more vitamins and stuff, just look on the back side of the can for the ingredients, etc. dependning on the size of can, prices go from 150 to 1000 pesos. Sold in every supermarket or colmado.