at least a long stories tnx.
http://storiesonline.net/s/56952/the-collapse-a-new-beginning-book-1 (and yes, there is a book 2)
http://storiesonline.net/s/43310/aftershock (localised apocalypse)
http://www.grynenbayritpublications.com/index/tales/aftershock/_default.html (the same story, re-edited)
There's also my Great Death series. The first book, Love and Death During the Great Death is strictly apocalyptic, while the sequels are all post-apocalyptic:
Love and Family During the Great Death
Grappling With Survival
Seeding Hope Among the Ashes
Note: The entire series is 'average guy faces the end of the world' and are not about doomsday preppers fighting others.
John Ringo's "Black Tide Rising" Series (5 books), Baen.
Try cmsix's "Nanovirus" story set. Many of his stories are incomplete but those are mainly finished.
There is a complete version available at the following link:
The original version is titled as such and complete. In my opinion it just illustrates how easy it is to reopen a finished story and add to it in online writing and publishing. Compare it with the version available on SOL to see the risks.
edited to add: The new material in the story should have been placed in a sequel not added to the original and left unfinished.