So can'd a local Kawasaki dealer get ahold of one?
Without a part number, eBay is the only source of leads that I found:
Here's a seller who has a Kawasaki sump gasket but not for the KZ650. Click the seller name to contact him to ask him if he can get one for your KZ650.:
http://cgi.ebay.co.uk/Kawasaki-GPZ50...QQcmdZViewItem
And here's another eBayer selling another type of gasket for Kawasaki...he, also, may have a connection on the sump gasket:
http://cgi.ebay.com/Kawasaki-KZ650-1...QQcmdZViewItem
Not a for sure answer, but hope it helps.