There are hotel brokers who buy up blocks of rooms ahead of time (at low rates) and resell them. One such outfit that I've used before is at
http://www.hoteldiscounts.com/ It used to be HRN; now it's part of Hotels.com. But the service is the same. They still buy blocks of rooms ahead, so they might have rooms on sold out dates. Quikbook
http://www.quikbook.com/ may also be worth trying, though I'm not 100% sure if they can get "sold out" dates. There are some others out there as well, I'm sure.