Just found out about a terrific resource for disposing of your old mattresses if you live in California, Connecticut or Rhode Island :
Visit http://www.calrecycle.ca.gov/mattresses/ in CA and/or https://byebyemattress.com/ to find a location near you. They appear to list recycle locations in many other states too, there is a possibility they have similar local programs. Check the websites for facilities, open hours and restrictions (3 per household here). Please let me know if you know of similar programs in your area.