You actually can use Unity assets in UE4, you need to install Unity, import the Asset, and then go to the asset in the folder and find the FBX file … can then be imported in to UE4. You may need to check the licenses on the Unity Assets though … some licenses say only to be used in a Unity project.
site explains how to import from Unity to UE4: ://creategamesfromscratch.blogspot/2014/05/prototype-sci-fi-environment-import.html
site has some cool links for assets: ://creategamesfromscratch.blogspot/2014/05/next-steep-prototype-assets.html
I have also used the following sites for assets:
Arteria3D
3DModels - Textures (formerly Dexsoft Games)
And then of course the locations that the other posters mentioned.
As the other posters mentioned, you will need to learn a 3D modelling program eventually, I am still have great success with Milkshape 3D, but I am slowly teaching myself Blender.