Can somebody please tell me ....(loot drop question)

...exactly where and off of what Mobs are dropping these kind of Bows with max damage's and mods:

Vampire
Half Moon
Frostbite
Short
Dead
Flat


I have more time than credits to spend and I would like to loot some on my own. but i can't seem to gain any info as to where (and off whom) anything decent drops.

Thank you Kindly in advance

/bump ty