I have been busy again in the DU Map Kit in UE 4.18 and I had previously downloaded a free sample of textures from the Unreal Marketplace which were from a site called www.gametextures.com.
The site has over 3500 materials in their library, and the materials appear to be quite high quality. The texture samples are 2048 x 2048 pixels, and the materials are made up of a few of these samples. Generally they have a standardized description of what the naming conventions for the material creation as well.
I should say that they don't provide free downloads, but for like $19 a month you get 40 items which you can download multiple file types of, and download as many times as you want. (40 materials basically a month)
I have so far downloaded a few and I have not been disappointed at all.
I also want to show off some of the textures in use in one of my level projects as well, The following image is a lit (flat lighting plus a few texture lights) room, created in the DU Level kit: