I recently replaced my 3 Bosch House batteries
while on the island of Armorgos in Greece
After a bit of searching on web I found a supplier that had a website that was in English
The supplier is Tserkezidis Batteries
Their prices are very competitive
They have a good range of flooded, AGM
& Gel batteries
They are based in Athens and will send your batteries to any island for only 9 euro each. The batteries arrived in 2 days.
The owner Vasileios speaks excellent English and provided excellent service by ensuring the batteries had arrived safely on the island.