Search by the theater nearest you that had the showing the last time
Look for movies at that theater starting with S
It will say "special advance preview may 5" but the show date will be may 26, 10 pm
Buy tix now, they're going fast!
(argh that this isn't at cantstopthesignal yet!)
(thanks to tips on whedonesque)
EDIT: PS: This comment at Whedoneque has links to several other cities through Fandango:
EDIT: Now a friend tells me that the way it is on Fandango has shifted. Now when you browse S movies, it just shows "Special Advanced Screening" and no indicator that it's Serenity, but the only date that pops up is May 26.