Tube patching glue (vulcanizing fluid)

I saw a friend riding the other day, and we were discussing what to
carry on a ride.

He mentioned, "A patch kit, and two tubes of glue. One that hasn't been
opened, ..."

And I jumped in, "...and one that has dried out since it was used for
the last repair."

We laughed, and went on our way. But how does anyone keep the darned
tube from drying out (or actually evaporating, I guess) between flat
repairs?

And where can one buy extra tubes of vulcanizing fluid?