Their own website should have tickets available, especially this far in advance. Those will probably be Tier (upper) level only, but the Tier really isn't that bad at Yankee Stadium. If you want better tickets, I'd recommend Stub Hub or EBay. I've found some decent tickets for below face on EBay before.
I took the Subway to the stadium and bought them from a scalper right outside the stadium. Got two nice seats for $30.00/seat and this was in late Septemeber.
Go to Ebay. I found 2 tickets on opening weekend for $30. They were upper tier on the 3rd base side. By the end of the game, we were 20 rows behind homeplate when A-rod jacked a grand slam. The ushers don't really enforce the seat rule unless the crowd is bursting at the seams. It really depends on the state of the team in August though. Unless they start making a run, you should have little problems finding some decent seats.
