Nebula is discontinued from normal production.
The ESP Nebula was the disc for the 2008 Discraft Ace Race. It is a smaller diameter mid-range driver that has a max weight of 174g.
If you like the Buzzz, but need something more overstable, then this is your new go-to mid-range! The Nebula has a very similar feel to the Buzzz in the hand but is more stable in flight. Maybe they should have been named Buzzz OS. It will hold true into a strong headwind where other mid-range discs will not.