They've been hard to get for ages, and generally I'm not impressed with the quality of CND abrasives anymore. My last lot of white blizzards for instance are glued together wrong so one edge over hangs, the other doesn't all the way to the edge. Their Koala's are not great either.
They do a cheaper versionof the Outblack in a different part of the website, iirc they're in 'disposables' or salon essentials rather than under the Abrasives heading. They're not as good but not a bad replacement, they also have a cheaper koala version. I'll see if I can find a link for you
They're cheaper version is this reference - PRODUCT SKU 140-63 - unsurprisingly they're out of stock too, I'm guessing it's a supply issue.