I know there are some threads on this already but they are pages long and need to be wade through, so here's a list of best places to farm for the elusive shards which i found best for farming.
3* Red and Purple (by far the most common ones) - Road to Run and Snake Eyes (the first 2 maps in Underworld either on Hard or Madness)
3* Blue - Toys of Death and Eagles Nest (Battlefield on Madness)
3* Yellow - Road to Run, Snake Eyes and Wayward Path (Underworld on Madness)
3* Green - Heart of Darkness (Battlefield on Madness)
3* Rainbow - Toys of Death, Shell Shocked (Battlefield on Hard and Madness) and Road to Run, Snake Eyes (Underworld on Hard or Madness)
Obviously there are other maps where they drop but from my experience so far these maps seem to have the best drop rates. I will edit this post when I see other maps with better drop rates.