Mixamo.com is one of the biggest source for free (almost) characters animations. Unfortunately its root motion support works really differently from the Unreal Engine 4 one, so both the skeleton, the mesh and animation must be modified to work.

It is a pretty heavy (and manual) task: you need to add a new bone to the skeleton, fix bone influences and update animation curves for the new bone.

The following tutorial shows how you can automate it with a simple python script.