Wow. Never knew there were so many supposed styles.
Not sure I agree with everything he's categorised though.
I mean Rave as Techno? It was a form of early hardcore last time I looked and it also appears in the hardcore category!
Must admit I usually buy at Beatport as you can listen to clips easily and the downloads are good quality not rips. Youtube's also a great source as most clips you'll find on music sites can be found in full on Youtube, allowing you to preview the full tune before you buy.
Music forums for the genre you like can also be good if they have a "Tune of the moment" thread or similar as then other people are posting tunes they've liked and it allows you to preview a lot of music you might not otherwise discover.
Other than that though, I agree with Adam, just buy what you like. Genre's only a rough way of finding it.