This is a paid asset, but now you can download Rifle Crouch And Prone Pro Free.
Detail this asset from Unity Store: Original Link
Rifle Crouch And Prone Pro v1.1
A set of 53 motion capture animations of crouching and proning for a character with rifle.
This set includes looped animations that can be blended together, transition animations and an example mecanim graph. It’s compatible with Rifle Animset Pro.
Video – Animation List
It does not include a controller.