baby formula?

so my baby is only 2 weeks and after being in the nicu, i struggled getting my milk out. I had so much milk while in the nicu. I would pumped about 5 ounces (both breast) each time I pumped. but when I got home, I stopped pumping as often. I was more worried about my baby and making sure she was breathing. After not being as stressed and my baby gained back her weight, I felt more confident in breastfeeding but she doesn't take my breast because she got used to the bottle. She does have a favorite breast and it's the only one she takes when she wants though. I still pump but the most I get is 2 ounces and only at times. I usually get less than 1 ounce. it sucks because my goal was to breastfeed but didn't count on having a hard labor and baby ending in the nicu. I got so stressed out. I decided to keep giving her formula until my milk supply comes back ( if it does) I give her what I have and just supplement with formula. however, she is starting to get constipated and idk what formula to give her. When i give her my breast milk, she poops fine but im not sure how long that will last. so i need something that she will use more of. I was researching and came across Babys Only formula. has anyone tried this out? if so, how did you like it? I am open to any advice and other formula brands. I just don't want her to be constipated anymore.