I'm not familiar with how things work in Greece but some things I'd consider investigating.
Does Greece have a national poultry association? Here in the USA that's a group that defines the requirements for purebred birds. If you can find them try contacting them.
Contact your agricultural ministry. Here, we have the Extension Service that has a local representative that gets involved with local farmers and businesses. They are often a great source for information.
Search for a Welsh Harlequin breed club. Here in the USA we have many clubs dedicated to specific breeds. You might get lucky.
Do you have a local Facebook (or similar) poultry group? It would be a great chance to chat with your neighbors.
Do you have poultry shows? If you can find one and chat with the organizers you might find some eggs.
Chat with people at your feed store. They may know something. Here, most have bulletin boards where you can post a message. Or are there other public bulletin boards where you might post a message?
That's all I can think of this morning. Good luck!