Well, maybe more sci-fi than “alien” but have you looked at Gendragon’s profile on sharecg? http://www.sharecg.com/pf/gendragon
Also spacebones has all kinds of different things that are handy and many sci-fi/alienish. http://www.sharecg.com/pf/spacebones
Those are my top two favorite folks when looking for things to use in bryce.
EDIT: Also you can find some curious things if you browse through the bryce section of rendo’s free stuff collection but there is a lot of stuff to look through.