Personally, I would never give my baby formula when they can have breastmilk.
As others have pointed out, formula is expensive and after buying a few cans you could have a decent pump. Since you only want a break occaisonally, I recommend the Avent Isis. I have one and I love it.
I've thought of a good analogy for formula: antiobiotics. Antibiotics can save lives, and when they are used appropriately they are wonderful. but the fact that every tom, dick, and harry prescribes them and uses them with every sniffle is BAD. they can cause resistance and that not only harms the person taking them, but it harms all of us b/c those resistant bacteria breed.
likewise, formula can save babies' lives, and for women who honestly cannot breastfeed, formula is a necessity and a godsend. but AFAIC, any woman who has been given the blessing of being able to produce milk for their babies OWES it to their babies to make sure that they receive the very best. someone here once said that breastmilk is every baby's birthright and I believe that to be true. I think it's the saddest thing in the world when a woman chooses to use formula without even trying and just lets that precious milk dry up. makes me almost want to cry for all the women who wanted to breastfeed and couldn't.
use of formula when breastmilk is available contributes to our FF culture, lines the pockets of formula makers and jeopardizes breastfeeding relationships by promoting the idea that it is "just as good".
okay, so maybe I sound a little militant, but I just really don't view formula as anything other than second rate food when compared with breastmilk. I suppose part of my "militant" attitude is b/c when i was preparing for the birth of my daughter, I knew I could find myself in several situations where formula would be pressed upon me, and I had to take the opinion that it was Junk Food in order to be sure I never succumbed to that temptation in the "darker moment's (fortunately it all went smoothly and I never was tempted).
anyways, back to you ! (sorry!)....can you ask the baby's father to help you buy an Isis pump? I mean, it would be to his benefit b/c then he could feed your child and take off without you around. and who would pay for the formula anyways?
whew...sorry for the soapbox rant!