It depends on your country.
The list is long, but your method is stated as per country.
Also, there are these ways as well:
>- From the Pokémon Trainer Club newsletter. If you do not already have an account, register on Pokemon.com and make sure to set your email preferences in your profile to receive the newsletter on October 20th. This appears to be the only way to receive a demo code in America.
- From certain online retailers by pre-ordering Omega Ruby or Alpha Sapphire or having bought X/Y in the past. For the UK this includes Amazon.co.uk, Smyths Toys, ShotTo.net or the Nintendo UK Official Store.
- From a 'Nintendo Zone' in participating outlets. This is usually your national gaming store such as GAME.
Source for the above points.
Hope I helped!