* The Song Remains the Same (http://archiveofourown.org/series/71495): John never married Mary, Dean never became a hunter, Sam never had a father. While Dean grows up in a typical loving family, Sam is raised as a guerrilla hunter cut off from normal society. They are thrown together for the first time when the yellow-eyed demon attacks Dean's family. Dean abandons his life as a college student and musician and takes to the road with Sam. Now they are fighting side by side, searching for the demon that killed their mothers and trying to save John. (Long series, each story is self-contained.)

* The Fire In Which We Burn (http://locknkey.livejournal.com/177967.html): Sam's a hunter, raised in the business since his mother and brother died 22 years ago. But now he's having visions of a green-eyed man burning on a ceiling. He thinks he knows what happened to his brother when their house burnt and his life changed forever. He's wrong.

* Spark (http://archiveofourown.org/works/73000): Dean Collins is a regular Chicago firefighter. Then he meets Sam, a mysterious guy who introduces him to a world he never knew existed. But they both find you can't run from the past, even if you've forgotten what it is.

* Due Supernatural (http://queenklu.livejournal.com/193027.html): Constable Samuel Winchester of the Royal Canadian Mounted Police has come to Chicago on the trail of the killers of his father and, for reasons about to be explored at this juncture, he has found himself attached to one rough-around-the-edges Chicago flatfoot by the name of Detective Dean Harvelle, who is not as dissimilar as one would suppose. Even factoring in the quite literal demons haunting Sam's past.

* Shell Game (http://archiveofourown.org/works/571651): Sam and Adam Winchester know when they’re being conned. Problem is, this new guy Dean Milligan seems to really be their half-brother. Adam is unamused, Sam is unfazed, and Dean is inappropriate.

These aren't quite in the parameters, but might be of interest:

Dean starts out as a hunter, then stops:

* The Space Between You and Me (http://archiveofourown.org/works/288969): When Sam is fourteen years old his older brother leaves, giving up the life of a hunter. Ten years later they meet again by chance and everything changes. Dean's not the person Sam remembers and Sam's not quite ready to let someone in.

* Isolations of the Weak (http://archiveofourown.org/works/2294216): Sam believes Dean and his father have been killed on a case. Abandoned on his own, he tries to live the life they would have wanted for him, as a hunter. Meanwhile, Dean and his father aren't dead - but they think Sam is, and ultimately give up hunting to have normal lives the way they think Sam would have wanted.

Sam is a hunter, Dean is something entirely else:

* Obsidian - Raise a Little Hell (https://www.fanfiction.net/s/10535430/1/Obsidian-Raise-A-Little-Hell): Dean's a demon, trying to find redemption. Sam's a hunter who's trying to rid the world from evil. Dean's kidnapping the hunter from his deathbed to heal & use him for a ritual that promises him paradise.

* In Arcadia Ego (http://archiveofourown.org/series/15311): Sam is the only child of the great hunter John Winchester, but life under his father's thumb is miserable for him. When a scholarship gives him the chance to escape to college, he seizes the opportunity to leave the grim world of hunting behind for the promise of a normal life. But when the same unknown creature that killed his mother year’s earlier murders his fiancee, Sam, embittered and heartsick, returns to his father’s side. John seems strangely reluctant to have him there though, and soon Sam is hunting on his own. While stalking vampires in the central states, he is forced to strike a deal with one to save the lives of hostages. Now, simple survival might be more than he can manage, and there are even darker things looming on the horizon…