How do you get shamin I can't find him?

First to get Shaymin you need to have beaten the game. After you've done a few things in the town and have the ability to switch leaders do some jobs. After you do a lot of jobs go to the Cafe. There should be a couple of Pokemon in front of it. After they get done talking they'll go inside. Just follow them in you'll see a crowd of Pokemon. Spinda will talk about a place called Sky Peak. There will be talk of a hidden Shaymin village. Now you have access to the village. The first time you climb the mountain you won't be able to get Shaymin on your team but when you go back you should be able to recruit it. Not 100 percent sure on that last part if you can't get it on the first time since it's been awhile since I've played. Either way you can't get Shaymin till you get access to the village.

After getting far enough in the game You'll have access to Shaymin village...complete the story part for it, then go back and go to the top again and Shaymin will join you.