Know-How Exchange : E-Marketing

Assuming you're a non-profit with low budgets
Assuming you have an up to date website, clearly presenting your organization,
Assuming your message is clearly stated on your website
Assuming you have an online form on your website to hear from members or.... volunteers

Monster would be one
Your own website, another
Find a blogger, pleading your cause and have him talk about working for your organization
Post your openings in discussion forums that you usual use to post your messages
Use your LinkedIn profile
Post it on your FaceBook profile
Use Twitter, simply saying you heard a bird sign about openings at your organization..

Some low cost ideas

Mikael
B2B Social Media Marketing

Three ways that I will list:

Craigslist-You can post a listing of what you would require the volunteer to do, and the contact information as well as any other information that may be required.

Facebook- You can message your friends, post on pages, or utilize facebook chat to talk to people directly.

Twitter-'tweeting' that you need help may be extremely effective considering that a lot of people may search to help people :D.

Other areas of help would be elance, YouTube (create a video), or look in forums directly related to your demographic. Hope this helps!