What to do if newborn won't change bottles?

Our newborn is formula fed. My wife was on medication that could no longer allow her to breastfeed. Baby is now 2 weeks old. She has been taking similac bottles with the disposable nipples. But now we are out and have to switch to the powder. She has taken the powder, it's the nipples we are having a problem with. She won't take anything else. We don't know what to do for her. Any ideas for concerned and worried parents?