How do I get a Coupon for the Lumiose City Boutique?

The Discount Coupon was available via Global Link until March 3, 2014 (thanks to @Jerry for finding that one out). If you got it until then, it will remain on your game until you use it or delete the save file.

For reference, here is a description of how it was done, which might apply to similar future events:
To get it, you simply need to create an account on 3ds.pokemon-gl.com, log in, register your game (use the in-game Game Sync feature to create a Game Sync ID) and click the right picture in the PokeMileage Club. The next time you use Game Sync, you will receive a Discount Coupon from the red Lady right of the PC in any Pokemon Center. You also get a confirmation on the PGL site once you've unlocked it.

After searching the seribii itemdex and browsing Google search, I have found that this item does not exist. It may be released in a patch, though this is doubtful.

Sources: http://serebii.net/itemdex/

Update:

It seems this item does now exist, obtained through a game on the Global Link site and transferable to your game, it is good for one transaction of various items at any boutique. While the item does exist, the game was a limited time deal and has since expired. It is possible it will be given out in other Global Link games later, however.