B.W.
So is she still nursing? If she is, then keep nursing, forget bottles and formula! Breastmilk has more fat than formula, and its absorbed and used better by her body. Nurse her one side only, don't switch breasts, that way she can get the fatty hind milk she needs. Rarely is a baby too underweight by nursing, unless baby is only getting foremilk from mom switching breasts mid feed.
ANYWAY,I agree with the others, get a sippy. My boys went right from nursing to a sippy around 6mos and weaned between 18mos and 2yrs from breastfeeding, no bottles. Worked out great.