Suggest you check out
www.workamper.com. They have a magazine and a online service called subscription plus that has various job opportunities, including a volunteer site for non-profit organizations which is available at their online site. Workamper used to be very objective in their listings and relationship with worker and employers. The original owner has sold the Workamper operation and the site is no longer the wonderful objective site it used to be. The current owner seems to be wimping out in his dealings where there are employer problems. However it is all there is right now and should be helpful for your needs. If you are Escapee members, you can also find listings at their website.
Wife Bonnie and I are workampers now for 10+ years now, happy to assist you if you have any questions, contact us at johns-email at earthink dot net