Going back to work... Help!!!

I will be heading back to work the 27th but my LO will be starting day care on the 20th. She has a good source of frozen milk but I want to get atleast 1 thing of formula just incase they need it at the day care. Any advice on type or brand? She's been 100% breastfed and I want to keep it that way but I also dont want her to miss any meals. She will be 2 months tomorrow!!