Congratulations on breastfeeding your baby!
I think the real question has more to do with why you want to start supplementing with formula. Whether the reason is nutritional, logistical, or anything else, you should do a little research on how to resolve that issue first, because once you start supplementing, your supply begins to diminish. Also, formula isn't as healthy as breastmilk. I took one look at the sample can that Similac sent us, and shuddered: "corn syrup solids" is the first ingredient on the list.
So, for reasons like these, I advise that you try other answers to your issue first; give any one or combination of possible solutions a good trial before resorting to formula. Your baby will be healthiest and best satisfied/fed through continued exclusive breastfeeding.
If the issue is that you want a little freedom, simply pump and store your pumped milk for other caregivers to feed her.
To get helpful answers on whatever other issues might be causing you to turn to formula (ex. not feeling like you're producing enough milk? Lots of easy ways to fix that!), check out La Leche League International's forums and other resources at llli.org. It's great to have the extra support, and you'll find expertise on breastfeeding that you won't find anywhere else.
I suspect that you want to supplement with formula because your dr., pediatrician, or a well-meaning friend or family member gave you inaccurate information, which is rampant when it comes to breastfeeding. You wouldn't believe what two doctors told me. One said, "some babies just don't tolerate breastmilk." Well, sure, maybe some babies don't tolerate the milk lactose from the dairy Mom eats, or allergens in the milk that the mother just needs to eliminate for a while (some babies outgrow allergies by 6 months of age). Or maybe they don't tolerate improper feeding practices, which I was mistakenly engaging in, leading to oversupply, making my baby really fussy! But babies are meant to tolerate breastmilk, or we wouldn't have made it this far as a species. The other doctor told me I should never have started breastfeeding because my breasts will forever sag because of it. What he doesn't know is that some slight droop can occur at weaning, but is completely temporary.
These are only two examples of the ignorance surrounding breastfeeding, coming from people that have barely learned the first thing about breastfeeding, and people think doctors know everything. Sure, they know a lot...about what they've specifically been *taught*. Go to La Leche League online, and even drop in one of the meetings of your local chapter. The support is tremendous. And they're so much fun!
Keep up the wonderful mothering!
L.