EditorUtility.OpenFolderPanel

Description

Displays the "open folder" dialog and returns the selected path name.

Additional resources: OpenFilePanel, SaveFolderPanel functions.

using UnityEditor;
using System.IO;

public class OpenFolderPanelExample : EditorWindow
{
    [MenuItem("Example/Load Textures To Folder")]
    static void Apply()
    {
        string path = EditorUtility.OpenFolderPanel("Load png Textures", "", "");
        string[] files = Directory.GetFiles(path);

        foreach (string file in files)
            if (file.EndsWith(".png"))
                File.Copy(file, EditorApplication.currentScene);
    }
}