I remember Erin of Erin's Hamsters was living in Cyprus (not sure if that's still the case)and having the same problem finding bedding. She was getting Aubiose shipped to her (that is hemp bedding). I am not sure where from but rat rations in the Uk ship to many countries and they sell aubiose - as it's sold in a bulk amount it will last a long time. I am not sure how expensive the shipping is though.
https://ratrations.co.uk/substrates-...d-hemp-bedding
At the bottom of the page is a link "Delivery" which has some info about delivery but doesn't specify how much it costs to ship overseas - it does say you need to be there to sign for it. To find out how much it costs to ship to Greece, you could put the bale of aubiose in your basket and go through the checkout process (but not complete it) and it should tell you how much shipping will be. Or you could click on "contact" at the bottom of the page and email them via their message option and ask them
Another option - check out any local horse supplies on google and see if they sell hemp bedding. Also any reptile supply places - they sometimes sell aspen which is safe to use.
While googling hamster supplies in Greece, I came across this
You have wild hamsters in Greece - might be an interesting day out some time, hamster spotting! Although I realise Greece is a big place!
https://ypte.org.uk/factsheets/hamster/print
It would take some time but you could just make paper bedding by tearing up a lot of plain white toilet paper into strips - it would be both for bedding and nesting.