Hi A.,
My guess is that you are going to get a variety of answers on this. My son was breastfed and I supplemented with Target's store brand of "regular" formula. (I forget what exactly they labeled it, but it was *not* the soy formula or the formula for babies with upset tummies.)
Whatever formula you choose, I would introduce it now, before you leave, so you know that your baby will tolerate it. On Day 1, start out with 1 ounce of formula in each bottle, on Day 2, do 2 ounces, and so on, until you've reached the breastmilk/formula ratio you expect to your baby to drink while you are away.
One tip about mixing breastmilk and formula: Put the formula and water in the bottle first, BEFORE you add the breastmilk.
Good luck!