You can have a look at New Scientist's web site http://www.newscientistjobs.com/splash.action and they have job vacancies and PhD research available there. Much like all PhDs though, you can always just write to a potential supervisor in an institution/area you're particularly interested in, but judging by the fact that you're looking for US/AUS/CAN proects you're not too bothered by the place you go.