The Mosasaurus Amber is very easy to find. You have to load up the level “Main Street Showdown”. The first part of this level contains the Amber.
When you load into the level you will see an InGen access panel. Open this panel using a character such as John Hammond.
This will open up a door nearby. In the next room there is a camera that will detect anyone near and cause the door to close, blocking access to the next panel.
Use a character who has the Camouflage ability to sneak past the camera. Then swap to a Jurassic World employee who can use the panel (such as Claire Dearing).
The glass will then smash and the Mosasaurus Amber will fall to the floor where you can pick it up. Enjoy customising your Mosasaurus!