If you are from an EU country: Use your SIM from home. After the termination of roaming fees, using Greek cards is more expensive then SIMs from other EU countries. This is for sure true for Germany.

If you want to buy a Greek SIM, there are several places in Pothia, for example Lathourakis on the inbound street and OTE on the outbound street, both after the taxi stand.

Generally, make sure to disable data roaming on the eastern part of the island, you will very likely catch Turkish carriers, resulting in high cost.

Throughout the accessible western parts of the island, data receiption is quite good.