If you ran our Trial of Valor clear back on April 18th, you might have picked up a quest off Odyn at the end of the raid which asks you to collect 1,000 soul fragments. The quest is called "Trial of Valor: The Lost Army". The reward is the ability to collect four different recolors of the armor set which drops in ToV. For transmoggers, this is most desireable.
However, collecting the soul fragments is a long slog. I think the devs want you to, ideally, run ToV, but this raid is like Ulduar or Trial of the Champions or Heart of Fear. Once people do it, they aren't likely to want to run it again. It's like ToV was the stopgap raid on the way to better content. So, you can be forgiven for deleting this quest from your quest log.
I have, however, discovered an alternate way to accumulate soul fragments. Periodically, a world quest will pop up which asks you to repel invaders from Helya's forces. This screenshot is from Azsuna, south of the murloc area.
Killing these forces will net you soul fragments. As long as the quest is active, I get, on average, one soul fragment per kill. Supposedly, you can continue to farm these forces for more fragments, but an hour of farming after completing the quest got me gear repairs, greens and gray items. No soul fragments. I don't think it's worth it to farm after you've completed the world quest.
In Highmaul, south of the murloc area, just south of the basilisks, there will be a quest to kill 4 casters who are trying to summon Helya into the world. They don't net very many fragments, but you do get a couple. In Stormheim, behind the building which houses Trial of Valor and down along the coast, there will be a world quest where Helya's forces are taking over the village. This village is usually overrun by naga in between vry'kul invasions. As with the quest in Azsuna, I average a fragment a kill when doing this quest. And, as with the Azsuna quest, supposedly you can farm these mobs for more fragments after the quest is completed. I haven't gotten any fragments outside of doing the quest.
Just doing these world quests has my total to nearly 400 fragments. We're going back into Trial of Valor in August so I'm hopeful the drop rate on these is higher once you're in the raid.
If you're not interested in transmog, most likely, this isn't your cuppa. But if you are, these world quests will help you progress toward more "You have not collected this appearance" gear.