Good bet is united.com. See the United Mileage Plus forum here on FT and have a look at the FAQ sticky on how to use united.com in expert mode.
I can also recommend KVS Availability tool, written by another FT'er (KVS). You may find the following useful:
http://www.flyertalk.com/forum/showthread.php?t=319244
The download link is updated in the first post.
NB: Once installed, you'll need to PM KVS to get an access code.