What dates did you have to use? I have picked random dates from now until May and it still says no availability.I just bought some 2 days ago (for first week of August). I had to play with the dates to get it to bring them up though. And, it doesn't matter what you put in for dates because the tickets don't have specific dates on them when you print them.
That's interesting that you can see dates out in September. I've only ever been able to see 20 days in advance. I want to buy them for my trip starting August 14th and they only go up to August 1st as of today. Very weird site