Your vacation is not the time to change your cat's diet. You'll only invite a whole lot of digestive woes for your cat and messy cleanups for yourself.
Instead of shopping for your cat's food in an unfamiliar supermarket that may not stock it, bring a large enough supply from home to carry you through your entire trip. Try to stick to your home feeding schedule while you're traveling.
On the day you're going to be flying or driving, don't feed your cat for about 3 hours before you leave, just in case your pet gets car- or airsick. But if the trip is going to be longer than about 6 hours, make sure to attach a dish of food to the inside of your cat's carrier so she doesn't get too hungry (some airlines will also request that you attach a separate container of food to the top of the carrier).