Transcribe by ear, When Problems Arise and Bonin' in the Boneyard by Fishbone.
Moderate to Advanced Slapping going on in those songs, which will show you what can be done with slap instead of just octaves which is what most beginners start with.
No, and I mean No Higher Ground
Other Famous Slappers/Artists
Primus (nothing to hard, fun music)
Marcus Miller (some is fairly difficult, bass is high in the mix and it is mostly slap)
Wooten (technician, does an alteration of slapping which is wanky sounding)
You should view slapping as merely another technique to use and not something that has to be done constantly.