From the hardware store?

Easiest glue: Collect chunks of conifer resin. heat over fire in a small pan. Once its liquified mix 1 part crushed common charcoal to 3 parts wet resin. You've got a stickey glue. You can let it harden in the pan. When you need to use it, heat the pan and it becomes wet glue again. This is how native Americans made their canoes waterproof. It does have a charming odor.

Second easiest: Collect peices of animal hide. Dried tails,ears, and tag ends are great. Legs are good. Sinew and tendon are best. You are rendering collagen so you need very low fat and no meat attached. Dont use pig, bear, racoon, squirrel or rat-- too oily. Dont use anything decayed or that has been frozen. Soak them in quicklime (kiln fire limestone peices gives you quicklime) and water for 2 weeks, skimming the top, to remove the hair and sanitize. wash with water, then lime wash again. Dry thoroughly. Cut the material into strips. Boil peices in a double boiler until they are rendered to mush. Let them dry. Control the temperature and dont let it get too hot or cold. Add water. Well prepared glue peices will absorb the water and you have a liquid glue. Failed glue wont absorb the water.

Apply the glue to the underside of the toilet.