Shaymin was obtainable at Toys R Us via Mystery Gift in the United States between 2/8/09 and 2/14/09. After receiving the Oak's Letter in a PokeMart after an event, go to Route 224, the extension route from Victory Road. At the end of the route, there is a white rock with Professor Oak nearby. Talk to him and Shaymin will soon appear and run away, letting you chase it down a really long, flowery path. Eventually, you'll catch up to it and be able to battle.
Without cheating or trading, it's impossible to obtain.