Thanks for the clarification. Such a provocative question - everyone's 'believable' recs must be so different. Depends on what you can believe - for instance, six impossible things before breakfast?! :-)

I'll give this a shot - although I've read so far afield in the Supernatural 'verse by now, that I can't be relied on as an objective judge. . . .

Believability is one thing within the familiar confines of the show. But I think their characters and connection are sharpest if the setting is unusual (for them), and the stakes are high. Do they continue to act and react the way we perceive them thru the show? Do we recognize them as authentic, no matter the circumstances? (This is all too subjective!) *mutters*

Try these on:

The Incestuous Courtship of the Antichrist's Bride (fleshflutter)
http://fleshflutter.livejournal.com/102268.html

Sam Winchester and the Terrible, Horrible No-Good Very Bad Day (leonidaslion)
http://archiveofourown.org/works/181646

Stranger than Fiction (nyxocity)
http://archiveofourown.org/works/379457/chapters/619888

Candlebeck's Gone Again series - or any of Candlebeck's work, really
http://archiveofourown.org/series/9421

Of course, the touchstone that is gekizetsu's The Last Outpost of All That Is
http://archiveofourown.org/works/1786042

runedgirl's 143 Alice Grim Lane
http://runedgirl.livejournal.com/57767.html

And I really like ygrawn's work, especially her Seven Women and Sam series.
http://ygrawn.livejournal.com/15915.html

Oh, and for retired-after-the-apocalypse Sam and Dean, I like compo67's Chicago'verse
http://archiveofourown.org/series/46578