Red Devil Lye.
You can buy it in almost any supermarket. It should be near the liquid plumber type stuff.
WARNING!!!!!
Lye is something NOT TO BE MESSED WITH UNLESS YOU ABSOLUTELY KNOW WHAT YOU ARE DOING!
It is TOXIC and EXTREMELY CORROSIVE!
ALWAYS have a gallon of vinegar nearby.
Let me re-iterate that!
One SMALL MISTAKE and it will BLIND YOU IN A HEARTBEAT!
If you want/need soap, just go buy it now and put it into storage.
You can buy like a cubic foot of soap for 5 bucks or so!