You did look in the top of the case and made sure it isn't in there, didn't you? My 99 has a place where the knee bar fits in the top, but I don't know how it does.
I saw some knee bars on one of the sites on the net, and think it might be in one something like Sewing DR or have the RX name in it. I did find this one who is planning on having some made.
http://singeroriginalvintageproducts...arforsale.aspx
You also might check with a local sewing machine repair shop and see if they have any. Good luck. You have a beautiful machine there.