Ratties Adopted some rats!

Hey everyone! About a month ago, I adopted four female rats from a shelter. I'm new to rats...I bought them a huge cage, some recycled paper bedding, and igloo, a sleep box...everything! They are spoiled. But I am having a hard time finding lab block type food for them. They get veggies and what not every day, but when it comes to a normal staple food, the only thing I can find is food with the main source of corn. Does anyone know a good place to buy the food? They are currently eatting the food that they came with which is Kaytee forti-diet pro health. That food also has corn as the main ingrediant. Is that okay for rats because I know guinea pigs and rats have different needs when it comes to food. Any help is awesome! Thanks!