AwardNexus can do a daily scan and will send you email alerts on low DL availability. I've used it a few times. You have to buy credits, and each search uses up a credit. You specify specific flight numbers and the class (note, specifying business also covers first, so don't do business/first as it uses 2 searches).

http://www.flyertalk.com/forum/trave...ard-nexus.html