Missing Loot from Story
I remember that Carapace Glove Box reward was tied to Tangled Paths Episode Achievement. And because you can do only once episode Achievements, you get them only once by doing story.
So only way how you get other Carapace Glove Boxes you need to buy them with 1000 bandit Crests. Or if you are lukcy, you might get them in rare drop from the Greater Nightmare Pod or Lost Bandit Chests.
You get one box for doing the achievement.
http://wiki.guildwars2.com/wiki/The_Master_of_Peace%27s_Bequest
The Master of Peace’s Bequest
Reward for obtaining the Tangled Paths Episode achievement.
Contains rare gear, obsidian shards, and a choice of a pair of carapace gloves.
The next 2 you need to either get as a drop, buy, or do the PvP track to get one box.
http://wiki.guildwars2.com/wiki/Carapace_Glove_Box
Buying the glove box: it can only be bought after it is unlocked by doing Into The Labyrinth.
In some cases, the carapace box is tied to the completion reward for the instance, in other cases, you get it from the achievement for finishing the chapter (ie. the achievement that unlocks the other achievements for that chapter).
You get the instance rewards once per character, the first time that character completes that instance. But you can only finish an achievement once per account.
The reward for finishing Episode 6 on multiple characters is getting extra Black Lion Keys, rather than Glove Boxes. You will need to buy the extra Glove Boxes for 1000 crests, if you can’t find them in Bandit Chests/Greater Nightmare Pods.
That said, you paid your 200 gems for the episode to get access to the Carapace Gloves in any form. Without the episode, it’s impossible to buy them even for bandit crests.