I'm sure you found it but this site is very helpful in getting the schedules: http://www.gtp.gr/

Sometimes you can book directly with the company operating the route but don't worry you can also just show up at PIRAEUS port. You will find several agents open even at 4:00am to sell you the tickets you need. Buying day of also allows flexibilty for any irregular operations or weather. I've never taken a high speed on that long of a trip but be wary that high speed ferries are effected by the weather more than a regular ship. SNA1k is right about buying business or first class tickets to gain access to the forward cabin or lounge on larger ships.

Above all remember, its Greece don't be in a hurry. Some off my best times have been on the deck of a very slow ship.