I'm afraid that there aren't many social sciences postdocs out there compared to the sciences (especially at the moment with hiring freezes). There are various centralised schemes; in the UK, British Academy, ESRC, Leverhulme, Canada, I think it's SSRC; where you need the backing of a university and submit your own project proposal. Otherwise, for the ones where you work on someone's grant, you are probably seeing all that's out there.I assume you're on the mailing list for the appropriate disciplinary association already, that's the other place I see postdocs advertised?