How can our armed forces continually find such men and women?

According to news reports, because of Hurricane Isabel,

[a]t Arlington National Cemetery, soldiers who guard the Tomb of the Unknowns were given — for the first time ever — permission to abandon their posts and seek shelter, Superintendent John Metzler said. But they stood guard anyhow.

The answer to the title question, of course, is that they recruit in America.  And these men and women volunteer, and they serve, and they do these things because they're Americans.  They are simultaneously ordinary and extraordinary beyond belief.