The textures are located inside the Warcraft III CASC archives, or MPQ archives if using a legacy build of Warcraft III.
A tool like Retera's Model Studio can be used to browse and view assets in these archives. If you want to just browse the assets then you can use MPQEdit or CASCView which are the go to tools for viewing the archive file systems of Blizzard games. The texture path within the archive file system is listed in the model itself. Where as Classic models have just a single diffuse texture, Reforged models have a set of textures for properties like diffuse, normals, emissive, ORM, e.t.c. Be aware that textures representing normals and ORMs are not visible images so editing them is more tricky than painting diffuse.