I know that this reagent can be dropped by bosses or sometimes found in the Bazaar; what I want to know is this: are they available through Gardening or Fishing? Because I would 100x prefer gardening to farming a boss over and over.
I find Astral Shards by farming Sardonyx. From the teleport in Silent Market on Khrysalis, I search the marketplace and the desert. Hope this helps!
Those are comet tails lying on the ground in those places (the Shadow Palace is the best place for them because of the small area, by the way). Astral shards are usually dropped by enemies.
