Ninja fish to answer your question, it's found in locked crates, & is supposed to also be found in the arena master chests. But it's appearing that locked crates are the main carriers of the fossil. (many of us have ran the arena 1.5k plus with zero rate of fossils found in the master chests, and come to think of it, the drop rate on the vials seemed to diminish in the master chests as well.)

good luck on your hunt
Ava