Which rear tire? There are several decent options noted over at
the Ninja 250 FAQ; there's also at least one (the Conti Blitz) that's available in a suitable size and not mentioned.
Oh, and I assume you've called around to your local m/c shops? The reason I know about the Blitz is that I picked up a used one for $50 ($77.50 mounted) when I was on a trip out of state with my brand-new SD's on a UPS truck en route to my home address. The local shop near my parents' house happened to have it in stock, otherwise I would've been stuck with the cage for transport while I was there.