There are several suppliers of hearing aids in Heraklion
but I couldn't find any in Agios Nikolaos. Ear Plus
seem to be nationwide chains. Both have web sites that claim to work in English but Evikoon only seems to work in Greek. It is quite likely that they will be more expensive than in the UK as spectacle suppliers certainly are. My late wife got two Siemens digital 4 channel aids at Specsavers for the price of a single Siemens digital 2 channel aid.
I don't know if any of the suppliers also do hearing tests, like Specsavers in the UK, but if not it should be possible to find an ENT specialist
to provide a hearing profile for you.
It will take a bit longer than a few days in the UK as, after doing the hearing test, they need to order the aids that you choose and then program them to your profile when they arrive. They also like to do the programming in two stages. They program in a profile that is the correct shape but doesn't have the full gain. Once you have got used to that they reprogram them to full gain. If you do intend to use Specsavers I would recommend visiting an ENT specialist in Greece a week or two before you go and have any wax removed. Any wax present distorts the profile.