Trajans are only available in the Romans Storefront, which you can access in-game via the button with card packs on it in the top left of your screen, under your Rank and Seal.
Trajans are offered in a variety of bundles and can be purchased through a variety of payment methods that you can select throughout the checkout process. Trajans can then be used to unlock Daily Card slots and to purchase Card Packs, also available in the Romans Storefront.
If you do purchase Trajans, please allow for up an hour for the Trajans to appear in your account. If they are not credited within that time, please contact support with your Transaction ID and we'll endeavor to resolve things quickly.