It takes time to get rolling but is pretty good now. You can get some randomly when dismantling gear, also the cubes (red cube thing can't recall the name). 4 per character per day from banshee bounties. Some at certain ranks with vendors Zavala, Shaxx, banshee, drifter, and Saint in the tower. The helm vendors (currently just the war table). Xur in the treasure horde. You can buy 5 a day from rahool if you have legendary shards. You can get some on a dice roll from objectives tied to ghost mods. Precision kills in pvp, wins in gambit, and boss kills in stikes. There's also ghost mods in the same slot for prisms, upgrad modules, and more legendaries. Once you get used to the game, there's also higher tier content like legend/master lost sectors which can drop 1-3 cores and 1-2 prisms and a decent chance at exotic armor. Then the nightfalls, don't know the drop rates but hero for cores, legend cores/prisms, master and Grandmaster for prisms/golfballs (ascendant shards).