Drifloon
If you want to catch yourself a Drifloon, there's only one place you can catch it...Valley Windworks.
Every Friday, outside Valley Windworks, you'll see a Drifloon out of the grass. Just walk over and click A to battle it. It will be at Lv. 15 so if you want to weaken it before you catch it, try not to send out Lv. 60 Pokemon against it. It doesn't matter if you catch it or defeat it, another one should be back the following Friday. This way, you can catch as many as you like, but every Friday. ;D
Don't bother to try and see if you can find one in the grass beside Valley Windworks because the only way to find them, is to find it on Friday. So don't waste your time and just wait for Friday! :D