My supply has dropped significantly. I used to be able to pump 12-16 oz while at work which was more than enough for DH and sitter to feed her and now I am lucky if I get 4-6 oz. Therefore....we might have to top off with formula

I still plan on nursing her as much as I can, and still plan on pumping while at work so she can at least have something.
My question is...what is a good brand to go with? We have a ton of samples of Similac with DHA and something or another else so we're going to start with that but should we use a different brand? And do we need to use boiled water or buy bottled water to mix the formula with? Any advice would be appreciated! Thanks!