I've sold several re-written fanfics to True Romance and True Love magazines. They accept unsolicited stories (4000-7000 words).
I'll disagree a bit with Erin's comment about needing an agent. It all depends on the publisher. I have a manuscript with Harlequin/Silhouette and most of their lines do not require you to submit through an agent.
Writers Market lists fiction markets and their submission guidelines. Many markets have their guidelines on their websites.
I will agree with Erin that it's not for the faint of heart. I'd be happy to answer questions as well.
