If the PhysicsOverflow search doesn't work (admittedly, it's not very good, and google and bing are raising people's expectations), you can use PhysicsOverfind, a google-powered custom search for PhysicsOverflow.
It's a slight development over using site:physicsoverflow.org in google as @JiaYiyang suggested, as Dilaton and I are currently able to add synonyms to the search (e.g. searching for "superspace formalism" should yield results for "green-schwarz formalism").
For future access, this can be found below the footer, under the "Collapse Sidebar" button.