This is a paid asset, but now you can download PBR Terrain Textures for Unity 5 for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
PBR Terrain Textures for Unity 5 v2.0
PBR Terrain Textures for Unity 5
Carefully crafted for optimal visual fidelity, perfect for creating a wasteland environment.
Mostly 2048×2048 pixels highly detailed and perfectly seamless.
Textures are tuned for Unity 5 standard and standard specular shaders, also tested and tweaked for UBER when used on meshes. Terrain textures are psd have smooth map in the alpha channel, mesh diffuse textures are jpg, specular/smooth is provided too for use with meshes. Some 4096px samplers added for upcoming PBR terrain pack.