| Great question!
I, like many started out learning slap from hearing Louis Johson, Larry Grahm, etc.
Victor Wooten was the 1st cat that made me realize that slap is just a technique and a way of "attacking" the strings so why put limitations on harmony/scale choice etc.
Up until then most slap was mainly octaves, pentatonics, etc. Vic really opened up the idea of also incorporating arpeggios, more linear scale choices, chords, etc.
Neccesity is the mother of invention and in expanding what was being played using slap it also created more approaches to the actual technique itself such as double thumbing/popping,, etc..
Other than that I personally draw a lot from drummers and percussionists. | 
