I've never been into Supernatural so I'm not the best source for this, but I have run across some crossovers. Most of them are not spuffy though. Faith/Xander for instance, but not Spike/Buffy. And most have been either on fanfiction.net or at Twisting the Hellmouth.
That said, there is one I just found that has spuffy in it although it's an ensemble. It is by a very good author too, but one that doesn't do a lot of spuffy particularly so isn't seen much around here. Laure Alexander.
apok�lypsis (revelation) by Lara/Laurehttp://ladyoneill.dreamwidth.org/12898.htmlSummary: There's an apocalypse and the Slayers are being kept out of it. When they find out that Lucifer's risen and a bunch of angels want the world to end so they can have paradise, Buffy and friends don't take it so well. They team up with the only people in the world who seem to be fighting for humanity--the Winchesters, Bobby and Castiel--and drag in Angel and his gang. Mankind isn't going to go out with a bang or a whimper if they have anything to say about it.Pairings: Spike/Buffy, Angel/Lindsey, Dawn/Connor, Lilah/Wesley, pre-Castiel/Dean, hints of Robin/Faith, Willow/Kennedy, and pre-Gabriel/Sam but really this is Gen with existing or coming into being relationships
Timeline: Set after Supernatural 5.10. Post-Chosen and NFA for Buffy and Angel, but Angel Season 5 went differently in that, while the battle still happened and those who died died, after the battle, Angel remained in charge of Wolfram & Hart's L.A. branch. Also, I'm ignoring the comics.
Warnings: Language, hints of sex and slash, a bit of violenceETA: I haven't read it, but I did glance at it and it didn't read as well as most of Laure's other stories. I'm not sure why, but it doesn't sound like her writing. It is supposed to be her though. I didn't really like it though, it seemed too...something. But it is there if you want to try it.
