Scyther can be found at the National Park during Bug-Catching Contests. Pre-National- Dex, there is a 5% chance to find one at level 13-14. Post-National Dex there is still a 5% chance to find one but it's levels when found will be increased to 27-28.
The evolution of Scyther, Scizor, is gotten by trading a Scyther holding a Metal Coat.
Source: http://bulbapedia.bulbagarden.net/wiki/National_Park#Generation_IV and experience.