Your local dealer would be a good place to start, they might have a used car specialist who can add you to a waiting/search list.
Car auction houses might well have a similar service, or register with them to get their sales brochures. Our 500X was on a Fiat PCP and went straight from us to auction (Mannheim IIRC) when it went back so they would be worth a look. I dare say rental companies will sell off their stock on a regular basis via auction houses too, if any have 500X's over the usual Ford/Vauxhall/Toyota/Land Rover SUV's?.
Or try a car finding service like: http://www.palmdale.co.uk/ (just to say I've never used them nor have any affiliation and no doubt there are others out there!).