Cards are available through mission but they are also available through summoning.
We have the traditional summoning method and the Guild summoning page, both are under the same section of the game.
Be sure to check the page everyday, because the rare summon has one higher probability mob everyday. Which randomly changes during the end of the (server)day
Summoning traditional: Edit
On this side we have the "Honor Summon" and "Rare Summon", the honor summon is available when the player has reached 90 honor points or rarely in chests and the rare summon can be obtained by buying it via gems. Both are able to be obtained by completing curtain achievements.
Honor summons tend to have 1 & 2 star heroes and occasionally 3 star heroes (it might be possible to get more then 3 stars).
Rare summons can summon 3star, 4star and very rarely 5star cards.
Guild summoning: Edit
On the second page of the summoning sections you have the guild summon page. This page has multiple options such as, Heroes, Mimic Chests, Element summon, Runes and Items.
Heroes: There are 3 heroes available, all are 4 stars and costs 1600 points
Mimic Chests: Mimic 400points, Bat Mimic 800points, Dragon Mimic 1200points
Element summon: Summons a rare hero of the chosen element and costs 825 points
Runes, Multiple Runes are available for the cost of 450 points per rune
Items: Multiple items are available (star potions) which costs 40 & 160 points depending on the item