Editor Scripts can make use of Asset files loaded on-demand. This is done by using the EditorGUIUtility.Load function. This function looks directly at the Asset files in a folder called Editor Default Resources.
Place the needed Asset files in the Editor Default Resources folder (or an appropriate subfolder within it). Remember, if your Asset files are in subfolders, always include the subfolder path in the path passed to the EditorGUIUtility.Load function.
You can only have one Editor Default Resources folder within a Unity Project. This folder must be placed in the root of the Project directly in the Assets folder.