I uploaded some new simple billboard nebula addons for RCW49, MyCn18, and M2-9 to the Celestia Motherlode. They aren't listed on the site yet but are in the archives, here's the links:
http://celestiamotherlode.net/creators/davemc/RCW49.zip
http://celestiamotherlode.net/creators/davemc/MyCn18.zip
http://celestiamotherlode.net/creators/davemc/M2-9.zip
One of these days I'll get off my rear and learn to use Anim8or (not just shamelessly modify other's work) and make some more interesting models for these.
Dave