Try the epilepsy (http://ohsam.livejournal.com/tag/epilepsy) and seizures (http://ohsam.livejournal.com/tag/seizures) tag at ohsam for starters. There are also a few fills at a comment-fic meme (http://ratherastory.livejournal.com/201862.html) I held at my LJ at the beginning of the season which have Sam with seizures.

There's also spn_littlebro's fantastic Delicious archive: http://www.delicious.com/spn_littlebro/h%3Dseizures and

Also I have a couple of stories in which Sam has epilepsy and/or seizures: my After the End (http://ratherastory.livejournal.com/tag/after%20the%20end) 'verse, and my 2010 Big Bang, Nihil Inherit (http://ratherastory.livejournal.com/60075.html).

Hope this helps!