You could try to find a Greek community in your surroundings (wherever that might be) - if you're lucky, they might help you make contact to someone who plays this instrument.
I have no idea what the exact difference is between askomandoura and tsambouna (if there is any) but the latter seems to be more popular if I'm not mistaken.
Also, a Turkish tulum player might be helpful - the instruments are at least similar.
This facebook group might also be helpful:
https://www.facebook.com/groups/doublechanters/