You should be able to find the Legendary Birds at all times, assuming you have not either caught them or they have not fled from you (or vice versa). If they have fled from you, simply defeat the Elite 4 again. If this is your problem, you should be able to find them again. However, if you have already caught them, you will not find them again in the Power Plant, The Seafoam Islands, or Victory Road. Instead, you can find them, albeit extremely rarely, out in the wild. If you have already caught them, you cannot find them in their usual places again.