Here are some free resources that I personally use. All credits belong to the respective owners.
Frontal & Perspective iPhone X PSD Mockups by Ramotion
Perspective PSD Mockup by Virgil Pana
Frontal PSD Mockup by Alexander Litvinenko
Frontal PSD Mockup by Unite Theme
Frontal PSD Mockup by Konstantin
Frontal PSD Mockup by Danish Designs
Frontal PSD Mockup by PSDDaddy
Frontal PSD Mockup by Design Bolts
Perspective PSD Mockup by Super Crowds inc.