yeah Ezra, i was going to mention yucca as well
the list in the link at the top of the page (table 2)
mentions the bark of yucca schidigera, i was thinking more the roots of yucca glauca
apparently bayberry is used in soap making... wasnt sure if it was a source of saponins so i googled myrica pensylvanica and got a result for myrica esculenta
seeds containing saponins
another one i came across in the richter's catalogue is atriplex canscens (saltbush) ... leaves soaked in water
thanks for the link to that extensive list