I have the same problem too. I breast feed my baby 100%. I want to feed her formula when going out as it is too hard to breast feed her in public, but she doesn't like formula.
One of my friends suggested me to start by mixing the breast milk (after extracting them out) with formula and then gradually increase the proportion of formula from time to time. It would take about one week for the baby to convert from 100% breast milk to formula. I haven't tried that because I can't extract enough breast milk (theoretically there is more breast milk coming out if the baby sucks directly from mummy compared to extracting them out) to try this method. :cry:
For those mums who can extract enough breast milk out, hope this method will work for you.