Hi DD,
Yes, Castor oil is derived from the Castor bean. It is still readily available, the beans are not, due to the potential for misuse. I believe Castor oil was/is used as a laxative. It was also used as a replacement to the oil obtained from beaver glands. I do not know of a suitable synthetic substitute. As far as use in a lube, it has a low freezing point, so you would need to add a wax to help solidify it at room temperature. I would imagine it would be fairly good as a lube as it does not mix well with water at all.
Oil chemist in a former life, amazing what you still retain.