If you can get hold of a 1" or 1 1/8" aluminium (or even steel) seat post, straight type with narrowing to 7/8" top, then you can make your own. I did, using an old quill stem's bottom wedge and a easily made long bolt. Then you can clamp a standard ahead stem to it. I can have pics tonight if you want to, but check if you can get such a straight type seatpost first.