I think that the Kawasaki Ninja 250 is the only bike of its type available in the US. All the other US-marketed sub-500's are cruisers. There are other 250 sportbikes available in other countries (like the ZZR250 you mentioned), but not here. You may be able to import one. But I don't think it's worth the trouble, since the Ninja 250 is such a great bike, and it's so cheap, and parts are abundantly available.
Curt
Curt