Just come back from a 20 days-holiday in Greece and here's what I experienced:

About beaches, what a relaxed atmosphere! I thonged everywhere without a single negative reaction. To be more correct, there were almost no reactions at all except for some looks that I suppose every fit body inevitably gets on a beach. Anyway, only few reactions and all positive.
My favourite place for thonging on the beach has been the island of Naxos. The vast majority of beaches there are mixed, which means you can find textiles and nudes at the same places. For instance topless is the most common for women. Everywhere people are so relaxed. Because of its calm I'd really recommend it for novice thongers who need to train their self confidence with thong swimwear!
Anyway, I'm sure there are plenty of other islands with the same atmosphere. Probably all Cyclades.

About thong shopping, did you know that in Athens you can go in a random underwear shop, ask about male thongs, and the saleswoman will probably have some? I tried in four different shops, smaller or bigger, (by the way, that was the best way to practice my Greek) and I could find a nice thong in all of them! Half of my purchases were Greek brands, all with very good quality. It's something I never experienced before in any other country.
To be precise, it seemed like all shops I visited, except one, had only few male thongs left (on sale!). I talked about it with some of the shop assistants and they told me that some years ago they used to sell much more thongs for men; it was a common choice for Greek men just like briefs and boxers. Than it seems that due to economic crisis people started to be more conservative and thongs' sells started to decrease about 5-6 years ago. I told them not to worry cause from now on I'll buy my underwear there!