You can earn Fashion Coupons by doing the following activities in TERA EU:

Instance matching

Join the instance matching alone or with friends. If you're successful you'll automatically receive Fashion Coupons per mail. More difficult instances will earn you more coupons.

Battlegrounds

Fight on the battlegrounds. Fashion Coupons will immediately be sent to your inventory afterwards. The number depends on the battleground, as well as whether you were successful or not. All currently available battlegrounds offer coupons.

Hunt monsters in alliance territories

Defeat monsters in alliance territories and be rewarded with coupons. Attention: Only the BAMS lurking stealthily in the territories are so fashion-conscious as to carry coupons on them! Other BAMs and monsters don't have much sense of style.

Defeating world bosses

Even level 52-60 world bosses have been infected by the contagious fashion fever sweeping Arborea. On dying they will drop a certain number of coupons. Make sure you're the first to defeat them, because they take a long time coming back.

Exchanging reputation points

Agnitor credits and Noctenium Society reputation credits can similarly be exchanged for Fashion Coupons. You can receive Agnitor credits by participating in the Nexus. Noctenium Society reputation credits can be earned by completing quests in the alliance territories and defeating monsters there. Visit the merchants in the Zulfikar Fortress or in your alliance territory to complete the trade.