Crucible usually gives at least 1 per match. Heroic quests help with 2 maybe 3. Of course milestones. Just break down those items for the shards. 2-3 shards a piece
i mean, idk what you mean by "where to", cuz if youre just playing the game, they just...come. Do strikes, Crucible, PEs, treasure hunt, turn in local Rep tokens... i mean, the only way you can stop the flow of purples is to just wander around the tower and dance at people.
It's probably just how much time you put in man. These guys are out playing you in hours. One of the loot items and strikes isn't a bad plan to maximise your time though. I always have more shit to destroy after strikes than pvp.