Larry Graham, Louis Johnson and Victor Wooten (all great, all different) are amazing to listen to and watch but way advanced to begin with.
Don't over look the books, many good ones out there !
I am working slowly though these:
Slap it by Toni Oppenheim. Toni has some samples on his website
http://www.slapit.com/example.html
Slap and Pop Bass by David Overthrow - A guide to Modern Funk Techniques
Also, its all in the technique. You can spend months just getting an octave slap/pop to sound consistent.