0.9 is three stops, or x8. Ten stops is 3.0 or x1024. B+W make the only ten stopper available in screw-in fitting (which you need to prevent light leaks). £80 in 77mm.
Hoya make a nine-stopper, called the x400, which seems rather nice as it's coated (B+W isn't) and the UK importers have just started bringing it in again. I'm hoping it might be a little more neutral than the B+W which has a slightly pink hue. However, they told me they won't have it in 77mm until the New Year. I'm gonna wait till then, meanwhile you can buy one from the US.