Same as any other bike-specific search; owner's club, Ebay, OBM, wanted ads in classic rags. It's just luck how long it'll take.
Been one in the classic rags for the last couple of months for about £5000 IIRC but that's really flying a kite.
Be wary though, SSs were from 1961 -66, and it's common for people to tosh up 5TAs etc and call them SSs, and also for people to misdescribe other models (eg T100S 1967-9 are often labelled SS by the cunning ones).
One more thing, if you come across the earliest model with the one-year-only rear bath tub, make sure it's present and the right type 'cos they're similar to the later type at first glance. I doubt you'll EVER see a correct tub for sale though..