If there is a leveling card event... Why is there no event that sell card in shop to go along with it? How are we going to level up our cards? Thin air? And please don't throw one of those "Buy it from AH" at me...
The developers of the game sets what's in the item mall (shop) of the game. Aeria has no control over that aspect.
I'm pretty sure when you open cards and run out, there's a option to directly buy additional cards for coins if you want to buy cards that way.
But buying from AH is actually better since you can get them cheaper, even during event. There's always people selling cards. Jewel farm gives random card boxes too which some people turn and sell right away.
You can also get cards from tasks, VIP weekly reward, and events. There's alsooo an event at the same time that gives cards (fortification event). It may not be much, but every bit counts!
A lot of people already do this but... It's also a good idea to save your cards up until next event if you're worried about having enough. :3 Just save them up and upgrade during next card event. Level of cards for rewards seems to stay the same for every card upgrade event so you can know how to plan and save. Lot of people also wait until events to smelt rings and fortify gear.
Anyway, hope this helps. :3