My common sources of carbs are potatoes (microwaved, then chilled in the fridge, consumed cold), stone fruits, citrus, apples, berries, avocado, peanut butter, peppers, tomatoes, yogurt, fermented vegetables.
I dont get cramps but I do supplement magnesium glycinate or taurate daily you may want to look into that if you havent.
Ive fasted for multiple days and done cardio each day w/ no cramps, just sharing my experience.