Just as an update for European readers, the Ernie Ball casters described above, can be bought direct from Ernie Ball in the UK:
[url="http://www.ernieball.co.uk/p/amp-caster-set-deluxe-pop-in-socket"]http://www.ernieball.co.uk/p/amp-caster-set-deluxe-pop-in-socket[/url]