Source material for Waste World is rare.
Here are some links...
http://gamingghetto.blogspot.com/ (home of some of the material from Ranger Firebase Alpha)
There is also a Waste World Yahoo! Group:
I did not particulary like the alien races in Waste World and either created our own
or adapted some from other settings such as Alternity, Fading Suns or Planescape.
Here are some examples:
Fading Suns Alien Races
Other inspiration I have taken from books e.g. by Iain M. Banks (e.g. Feersum Endjinn) and
Gene Wolfe (e.g.The Book of the New Sun) and also from the RPG Legend of the Five Rings.