sometimes about now some tickets become available. Seats are block booked by agencies (coach parties etc...) there is a date by which they have to pay for tickets, any they haven't sold go back on sale - might be worth a look as I think it was soemthing like 6 weeks before so any time now..
I booked two tickets about a month ago for sunday night, and to be honest, there was precious little availability then.
I am sure some will come up on ebay at some point but you might have to pay over the odds for them.
ets - have you tried ringing the box office? Some of my job involves working on the Box Office phones for a theatre company, and I know that often we have availability that for various reasons isn't accessible on the website. Its possible that even if they have no spaces right now, they will know when some space is likely to become available.