Have you posted to Volunteermatch, Idealist and Craigslist? Have you posted to your local volunteer center? (call your local United Way for the contact information if you can't find it on Google.) Have you sent the opportunity to your network on LinkedIn or your social networking sites (Facebook, MySpace, etc.)?
And do you have a web site with complete information about this volunteering opportunitiy? (detailed assignment description, training required, commitment required, skills required, etc.?).