Here, this article will explain: Use Tri-Planar Texture Mapping for Better Terrain
You can use any texture, since the technique basically blends the texture from all tree axes. By using the WorldCoordinate3Way node, you can blend 3 different textures, or the same one on all three axes.
This one, Open World Demo Collection in Epic Content - UE Marketplace have it all for the outdoor environment, but it’s a bit resource heavy and takes time to load it up, but once you got it you can migrate some stuff out of this basket if you will. They’re free to use for all purposes with some brand limits.
Tileable landscape textures can also be found all around the internet, the best quality pry comes from purchased kits only, but for rapid prototyping just use anything you already have, even a checkerboard should do it until the artist gets you the fine textures. There are also nice marketplace items from talented artists worth to note, when you’re after high quality game ready stuff out of the box.