For those outside the YHA regiment, you can easily find and book them online. Before placing your booking, check out everything about the hostel, the type of room they provide, what they expect you to bring along - many do not provide toiletries or even towel.
There are many youth hostels in Greece. Despite its name, they are really open to guests of all ages, offering the lodger either a bed in a dormitory or a private room, usually without attached bathroom. Of the many only five are within the orbit of the International Youth Hostel Federation, or YHA, and three are in Athens, and one each in Corfu and Santorini.
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