I'm an Amazon junkie, so I would just buy it there.

Kawasaki OEM Thin Wall Spark Plug Wrench 18mm 92110-1111 https://www.amazon.com/dp/B00NB34NMO/ref=cm_sw_r_cp_apa_4WTwzbX7MGCK0

If you really hate that idea or need it right away you can try a small motor shop, such as for lawnmowers and chainsaws. Or the obvious, a kawasaki dealer.