Thank you again y'all!
I found a used book "The Best of Fritz Leiber"
via Amazon - thanks for that hint!
And then I decided to look at the wikipedia page for the story, A Pail of Air, and it had a link to a collection with the story
on Baen Books if anybody else wants to read it.
And... it was still a good read, despite time having gilded my memories. Definitely a keeper