Supplementing formula


My milk supply has dropped drastically in the last week or so and I'm starting to worry. When my baby does feed from the bottle he drinks 4 Oz at a time but when I pump I barely produce 3 per pump anymore and it's taking longer and longer for my milk to drop. I'm really worried because my baby is only 2 going on 3 months but if I want him getting the amount of nutrition he needs I figure we should start supplementing. Are there any suggestions on how to start? Enfamil is the brand my daycare carries so I figured I should start with that but are there better brands?