Tile Map XML Tutorial Tileset Textures

There are many textures that you can use for this, Tiled comes with some default map textures. You can find free assets on the web or in the Unity Asset store or use ones that you create yourself. A good place to look for free assets to use is opengameart.org and Liberated Pixel Cup(part of Open Game Art). Kenney Vleugels has some free and paid assets found at http://www.kenney.nl/. Jason Perry has some good ones that are a part of his Time Fantasy Resource http://www.timefantasy.net, He has some free assets that are compatible with the paid assets. Another good paid asset is in the Unity Asset store called 3D Pixel Tiles https://www.assetstore.unity3d.com/en/#!/content/14970, for $12.50. The good thing about this asset is it also includes the full 2D Pixel Tileset asset. You can use the 2D tile set to create your map in Tiled and the 3D meshes to display a 3D map in Unity. There are many 3D assets on the Asset store that do not already have a 2D tile set, you can always create your own tileset from the 3D assets. Make sure that you follow the license that comes with the assets you are using.
Copy the textures that you are going to use for test map into TileMapXML->Maps->Tilesets.

Tile Map XML Tutorial Index
Introduction
Initial Unity Setup
Tileset Textures
Tiled Maps
Tiled Map XML Loading in Unity
NUnit Testing
Tiled Map Format