Is it okay to supplement your breastfed baby's diet with formula
I would not recommend it. Supplementing can lead very quickly to the end of breastfeeding. It is easier for the baby to get formula from a bottle than milk from a breast. There is a different tongue mechanism at play. Not to mention nipple confusion, and overfeeding. You do not (unless you pump) measure how much breast milk your baby gets at each feeding, and supplementing can increase calories drastically, leading to obesity.
I only struggled with my milk supply with my first two and I believe it's because I didn't rest enough, didn't drink enough fluids and get enough protein or eat enough of the right kinds of food. I think sometimes the obvious is overlooked when you are a new mom and that's why there are so many breastfeeding struggles, that and the fact that many of our mothers didn't breastfeed and could not properly mentor us in this art of mothering.