The whole purpose of lemon juice in this recipe is to prevent the apples from oxidizing. If you do not have lemons on hand, you can do a quick soak in either citric acid water (1 teaspoon per cup) or even salt water for 5 to 10 minutes. If you do not have access to any of these options, try not to cut the apples much ahead of serving them. In a pinch, even tossing them into plain, cold water will keep them from oxidizing while you prepare the rest of the salad.