I know you asked for Buffy/SGA which I'm sorry but I've never actually even seen a Buffy/John crossover. But due the sheer, unmitigated AWESOMENESS of these two stories, I thought I'd rec them to you anyway. The stories are Far from Normal by Jakyl which can be found on the site Twisting the Hellmouth at http://tthf.me/h6YH and it's sequel Return to Normal at http://www.tthfanfic.org/Story-13331/jAkL+Return+To+Normal.htm. These two stories are so extraordinary they have over 663,000 hits between them. The basic premise is that Buffy did not survive her encounter with the First, but because the powers want to keep her as an ace in the hole, instead of letting her die again, they pop her into the alternative dimension where she had hallucinated she was in an insane asylum. Except this time she actually IS in the asylum, but completely lucid and also without any of her slayer powers. I won't give away any of the wonderful twists and turns, but suffice it to say she eventually ends up in the hands of SG-1 and Stargate Command and the awesomeness just keeps coming and coming. And then there's a sequel! If you want a well plotted, well written Stargate/Buffy crossover, these are the ones. Oh and Twisting the Hellmouth is a site devoted to Buffy crossovers of all flavor, so you might browse the site for the SGA crossover you're looking for.
There's jedibuttercup's Uncle John series (unfinished WIP), where John Sheppard discovers family he never knew he had, nieces Buffy and Dawn Summers. gen [Also at Wraithbait here]
Buffy/SGA crossovers on FF.net (http://www.fanfiction.net/Buffy_The_Vampire_Slayer_and_Stargate_Atlantis_Crossovers/13/1833/).
I know I've seen a couple at AO3 too, but the tags don't work there at the moment. An advanced search gave these hits (http://archiveofourown.org/works/search?utf8=%E2%9C%93&query[text]=buffy+and+john+sheppard&query[tag]=&query[title]=&query[author]=&query[words]=&query[hits]=&query[date]=&query[language]=) (if the link doesn't work, just do an advanced search and write "buffy and john sheppard" in the "Any field" field.
My two personal favourite Buffy crossovers are both John/Faith: 67 Days (http://lyl-devil.livejournal.com/41714.html) and Wound (http://thingswithwings.livejournal.com/16100.html).
If you're looking for "Buffy" as in the show then I haven't seen Sailor's Delight (http://jinjurly.com/redsky.htm) by general_jinjur in any of the rec lists above!
It is an Apocalypse SGA/SG1/Buffy/Night of the Comet crossover fic, which sounds a lot but it is brilliantly written. Dawn wakes up one morning to find she missed the comet from the night before but that everyone is dead. Everyone that is until she bumps into Jeannie Miller who is following a coded letter from her mysterious brother. It is told from Dawn's point of view but none of the characters who appear in this crossover are short changed in the writing.
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Date: 2012-07-16 04:41 am (UTC)no subject
Date: 2012-07-16 01:27 pm (UTC)no subject
Date: 2012-07-16 03:39 pm (UTC)I haven't read any of these, but Twisting the Hellmouth has 11 Buffy/John Sheppard fics here: http://www.tthfanfic.org/Category-3-4963/Pairing+John+Sheppard.htm
You could also start at TtH's Stargate page and click through via character filters or just click All Stories to browse through: http://www.tthfanfic.org/Category-1-17/Stargate.htm
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Date: 2012-07-17 08:31 am (UTC)no subject
Date: 2012-07-16 04:03 pm (UTC)I know I've seen a couple at AO3 too, but the tags don't work there at the moment. An advanced search gave these hits (http://archiveofourown.org/works/search?utf8=%E2%9C%93&query[text]=buffy+and+john+sheppard&query[tag]=&query[title]=&query[author]=&query[words]=&query[hits]=&query[date]=&query[language]=) (if the link doesn't work, just do an advanced search and write "buffy and john sheppard" in the "Any field" field.
My two personal favourite Buffy crossovers are both John/Faith: 67 Days (http://lyl-devil.livejournal.com/41714.html) and Wound (http://thingswithwings.livejournal.com/16100.html).
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Date: 2012-07-18 11:53 pm (UTC)It is an Apocalypse SGA/SG1/Buffy/Night of the Comet crossover fic, which sounds a lot but it is brilliantly written. Dawn wakes up one morning to find she missed the comet from the night before but that everyone is dead. Everyone that is until she bumps into Jeannie Miller who is following a coded letter from her mysterious brother. It is told from Dawn's point of view but none of the characters who appear in this crossover are short changed in the writing.
no subject
Date: 2012-07-19 07:13 pm (UTC)http://delicious.com/mirrored_illusions/buffy.the.vampire.slayer+stargate:atlantis?&page=1
Hopefully you'll find something that'll interest you.