I've never seen fork braces specifically for the Ninja 250 or 300. I'm not even sure how you'd go about making one.
EDIT: Looks like some Japanese companies make them, maybe the same thing as Bikerz products? I have no experience with any of them.
http://japan.webike.net/products/20124818.html
Another edit: after more reading, those are the same parts as what the brand Bikerz sells. Seems like the fork brace is the only part Bikerz sells that actually does anything other than look chunky. Chat with gallardo73 and see if he knows anything about the fork braces.