As wildly original as this story is, O'Connor didn't invent the title "A Good Man is Hard to Find." It's actually the title lyric of a song composed and performed by Eddie Green back during the time of piano rolls (1918). The song was a hit when it was first written, but became even more popular as a signature tune by Sophie Tucker.
O'Connor probably expected her readers to know the song, or at least the lyric that comes next:
A good man is hard to find
You always get the other kind
Yup: that certainly seems to be true in this story.
The title is also a line of dialogue; it comes into play when Red Sammy and the grandmother are talking about how no one is trustworthy anymore. Red Sammy makes the assertion that a good man is hard to find, and the grandma heartily agrees. She doesn't, unfortunately, know that she'll "get the other kind" in short order.