Oh well, looks like you might have to order them direct from MikroKopter in Germany. When ever I order replacement bits from there I always add a few low cost items like mounts, threaded pillars, etc to the order just in case. It reduces the disproportionate delivery charges that always spoil small orders.
It seems possible to get smaller 6mm ones from
polymax.co.uk/Anti-Vibration_Rubber_Mounts/B_Type_Anti-Vibration_Mounts.shtml
You could always stack them