The best bet is to look for one outside Greece.
Since the murenas are so rare Im afraid that you have to import one.
Germany seems to have the largest population of matras outside france.
Try mobile.de autoscout24.de or ebay.de ... there are always a few for sale.
Alternatively you should speak to Ruud at CARJOY in Holland. He deals exclusively with murenas, and always has several cars to choose from.
He is also very helpfull with any paperwork that needs to be done regarding the export of the car, and last but not least, you have a garantee that you wount get ripped off
good luck in your search