Use the "File dialog" from the advanced File IO palette.
-- Select "Existing dir" for the mode
-- Wire a single space character to the pattern (for example).
-- I assume the file name will be programmatically generated.
--> The file dialog will only show folders, because the file pattern never matches. (Of course, this is only a soft barrier, because users can still select "all files", highlite a file, then hit delete.)