Out of curiosity, what general areas are you finding a need for alternate ads? The only reason I ask is I have never had an inventory problem in any of the areas I have sites for so I am just curious where adsense is serving up no ads/psas...
Chitika can be a good alternate, I wrote a blog entry about how to use Chitika as an alternate URL for AdSense: http://www.admoolah.com/blog/index.php/2005/10/using-chitika-as-an-alternate-ad/