My KLX300 came with one. I'm not sure the KLX runs hot enough (stock) to need it, but it's nice that Kawasaki included it. It's located under the left number plate, mounted directly to the frame at two points. On a big spill it'll leak through the overflow tube (routed to discharge right above the exhaust) and hit the exhaust pipe, white billowing clouds everywhere!
I'd try Stroker Speed 760-948-2871 or even like Denis Kirk, Chapparral or White Brothers, they probably have it.
Matt