There are some older answers on my flair profile that you might find interesting! I also did an AMA about the Peloponnesian War last year.

I don't know of any online communities specifically devoted to this conflict, but in any case it's hard to trust the information you get on such forums (he says, ironically, in a post on an online forum). People don't always have the credentials or the reading necessary to give you useful information. However, our main sources for the conflict are freely available online:

• Thucydides' History of the Peloponnesian War (covers the prelude and the period of 431-410 BC)

• Xenophon's Hellenika (covers the period of 410-404 BC, and the continues on down to 362 BC)

That website also has many other sources from or about the period, such as the comedies of Aristophanes, the tragedies of Sophokles, Plato's early dialogues, and Plutarch's Lives of Alkibiades, Perikles, Nikias and Lysander.