Two options.
First Kyosho Europe still have stock of the wishbones, part number UM13.
Order them through either
Ben Racing or
Time Tunnel Models
Alternatively the wishbones of the Tower Hobbies ST-15 are the same shape as the Ultima
http://www3.towerhobbies.com/cgi-bin...?&I=LXWZ82&P=7
Most of the ST-15 suspension parts are interchangeable with the Ultima.