formula?

I've been struggling with breastfeeding. It's a combination of a poor supply and my little one doesn't suck vigorously enough to encourage my supply. Right now, I am nursing and supplementing with formula and pumped milk. I am thinking about we will likely switch to bottles overall because she's not gaining the weight she needs. She's doing well with the ready to eat Similac right now, but we'll probably switch to a powder formula in a few weeks. Any recommendations that work for you? Thanks!!