In a small bowl, combine the sour cream, 1 teaspoon cilantro, onions and garlic powder; cover and refrigerate. In another bowl, combine the corn, tomato, jalapeno, orange juice, oil, salt and remaining cilantro; cover and refrigerate. Just before serving, spread 1 teaspoon sour cream mixture over each tostada. Using a slotted spoon, top each with a teaspoonful of corn salsa.