It's available and working fine. You're probably looking at the beta extensions site; just go to the original one. However, until it's all settled, Safari 9 won't auto-update extensions from the non-beta site.
I don't see ANY ad blocking extensions available from Safari 9 in El Cap.
But, I already had 3 different ad blockers installed in Safari from before I updated to beta. All appear to work. But, they were previously installed, and none are showing as available for downloading through the internal Extensions tab.
The previous posters probably have the best idea. Try getting directly from the Ad Block site, or whatever others you might want to try.