Soaps are made from which plant source?

Soap is made of oils and fats, of any plant or animal source, by a process called saponification, where they are reacted with lye (NaOH or KOH) and converted to soap, which is a sodium or potassium salt of fatty acids from oils/fats. While some time ago, mainly soap production was based mainly on animal fats, in the beginning of the 20th century, palm, palm kernel and coconut oil became the most used oil sources for industrial soap production - majority of toilet soaps you find in your drug stores are made mainly of these. Olive oil is the main base for castille soap and Alep soaps.

Soap can be made of any vegetable oil, so the answer would be - any plant that gives oil in sufficient quantities. What differs are the final characteristics of soap (foaming, cleansing, conditioning, hardness...), which are a function of different type of oils.