The best source for anything Greek, not exactly the most detailed in terms of an outlined history, but it gives great insight into the politics, culture, arts, and economy of pretty much every single section of Greek history and has plenty on the Byzantine Empire (early, middle and late):

http://www.ime.gr/chronos/en/

Click and choose my friend, the only thing is that since this is the english translation of the site it still lacks some of the smaller parts that the original version in Greek has, but it isn't missing anything major.

Enjoy. :)