As it was explained to me, the soap people made themselves in the old days was so much better than today's, a bath could last you weeks, or even months.
Obviously, plain old soap bars will not suffice to make a bath last for weeks or months. As I said pretty clearly, and you must have read, you cannot buy the good stuff.
You may try finding soap that will make a bath "last" for weeks. If you do find somebody claiming that, and actually give them your money, that is on you. I promise you will not get any of it.