EX250F key blanks
If you need a new key for your bike, there is one good choice, one hard-to-find choice, and one expensive choice.
OEM Kawasaki key blanks
The "gold standard" key blank is, of course, the OEM Kawasaki, part number 27008-1173. Dealers typically ask well over $10 for these. You can find them cheaper at places like Bike Bandit and Ron Ayers, but with minimum shipping charges they're the same or more than a dealer. The OEM key is nickel-plated brass (as are all the other keys mentioned here) with a plastic-coated head.
Ilco X103
The least expensive and most common key choice for the EX250F. This is also marked as KA14 and is what you find when you look up the bike model in the Ilco book. Use this if at all possible.
The Ilco USA X103/KA14 is slightly shorter than the OEM key and is all-metal, with a head profile significantly different in shape. The length reduction is in the head end of the key. The portion of the key shaft that inserts into the lock cylinder measures the same as the OEM blank.
Note: There are two Ilco lines: Ilco USA and Ilco Italy (aka ORION). The Ilco Italy brand has a key marked KA14, but that key is not remotely related to any Ninja 250 key.
Note 2: Taylor made the KA14 as well, but Taylor is no longer in business. Some of their blanks may still be around, though.
Silca KW12BP
The Silca KW12BP has a black plastic head like the OEM key. It is 3mm longer at the top but is functionally identical. Unless you held it against the OEM blank you wouldn't be able to tell the difference. The KW12BP is hard to find; you'll probably have to look for it online. Even then it will be hard to find. And it will cost about twice as much. Try My Security Pro.
Avoid Ilco X266
One Ilco blank that some people have used is the X266, also marked KA16. This one is not listed for the Ninja 250 but rather the Concours. This blank is also all-metal, but its shape is the same as the OEM key. The problem with the X266 is that there is a small ridge on each edge that prevents the key from being fully inserted into the Ninja 250 lock cylinders. The amount of interference is fairly small (less than a millimeter) but that is enough to cause problems. Without a minor modification to the key, an X266/KA16 blank cut to fit the ignition/seat/helmet locks will not work in the fuel filler cap.
Here's a closeup of the ridge in question:
You can see where the ridge interferes (seat lock cylinder shown):
The modification of the blank is to use a file or other tool to remove the ridge just at the end on each side. Only a tiny bit needs to be removed, not the entire ridge.
Key dimensions
The critical dimensions of the aftermarket blanks are identical to the OEM Kawasaki blank, with the exception of the ridges on the X266/KA16. Aside from the thickness and width dimensions of the portion of the shaft that inserts into the lock cylinder, the length from the tip of the key to the shoulder step, 20mm, is most important. The reason for this is that the ignition, helmet, and seat lock cylinders rely on the shoulder stop to determine insertion depth, whereas the gas cap has an internal stop such that only the end five of the six tumbler positions are used. If that 20mm length were to change, then a key cut for the other three lock cylinders would not work for the gas cap, and vice versa.