The fancy patterned Vivillon cannot be found in the wild and is available to all Pokemon X/Y players for free for a limited time.
Pokemon trainers with a copy of Pokemon X or Pokemon Y are now able to pick up a special Vivillon via Mystery Gift. The Pokemon Company notes it won't be available for long, so don't forget to start up your Pokemon game if you've left it sitting for a while since completing the game.
Vivillon is a Pokemon that have a different pattern depending on the region of the player. This giveaway includes a Fancy Pattern Vivillon, shown to the right. The Pokemon Company thanked all Pokemon trainers making trades through the Global Trade Station. The Fancy Pattern Vivillon commemorates the 100,000,000th Pokemon traded via GTS.
To obtain the Fancy Pattern Vivillon:
1. Connect your Nintendo 3DS system to the Internet.
2. Select Mystery Gift on the main menu.
3. Select Receive Gift, then Yes.
4. Select Get Via Internet, then Yes.
5. Watch as you receive Fancy Pattern Vivillon!
6. Speak to the delivery girl in any Pokémon Center to receive your Vivillon.
7. Be sure to save your game!
The Pokemon Company noted that Vivillon "won't be available for long." If you want it, make sure to do so today. Fancy Pattern Vivillon will not be available in the wild.
Source: Pokemon Company