Do you have any specific reasons for volunteering?

If it's for something where you need an official record (for F-2 visa application, for example), then it's best to find organizations who are signed up with the VMS site:  http://www.vms.or.kr/index.jsp

It's the official site that Koreans use when volunteering.  If you know a Korean who can help you, then you may be able to get them to contact VMS or look on the website.

If you are volunteering for reasons where you don't need any record, you can try this:

http://www.koreahow.com/2011/04/1208/

Good luck!