I can't answer your formula question, sinds I don't haven't heard of Enfamil.

But I do know places to buy babystuff.
Try Motherscare, in Noord.
Or PriceSmart near the airport. For this shop you need a discountcard, but even without the discount they have babystuff.
And you can also try Certified shopping mall.

I have a baby girl born 13 oct. 2004.

Enjoy your stay on Aruba!