most bearing and hydraulic supply houses have o ring cord and can make you one
this expensive kawi one is easy to find and works
http://www.ebay.com/itm/NOS-Kawasaki-Ninja-Engine-Cylinder-Head-43-7x1-9-O-Ring-88-08-ZX-KLF-ZR-Ninja-/310224530548?pt=Motorcycles_Parts_Accessories&hash=item483ad2ac74&vxp=mtr
this expensive kawi one is easy to find and works
http://www.ebay.com/itm/NOS-Kawasaki-Ninja-Engine-Cylinder-Head-43-7x1-9-O-Ring-88-08-ZX-KLF-ZR-Ninja-/310224530548?pt=Motorcycles_Parts_Accessories&hash=item483ad2ac74&vxp=mtr