Actually, if I remember correctly Ferrets are a true carnivore with a 4 hour digestive system. A cat, for example, would be a true carnivore with a 6 hour digestive system. .. so there have been instances where ferrets do not do well on cat food because they cannot absorb the nutrition they need from it before it's passed out of their system.
Most pet shops carry a blend of food for ferrets, but a high quality cat food should be ok in a pinch. You can try a wet mixture of canned food as well.
If you're in a pinch you can feed a ferret Iams Kitten food in the purple bag, only. I know many people who feed their ferrets this all the time, it's the only brand of cat food that they seem to do well on if you cannot afford or get actual ferret food.