Freecycle:

Freecycle is a grassroots, online network of local groups where people give and get stuff for free in their own towns and neighborhoods. It’s a fantastic resource for finding free mattresses, as members often post items they no longer need, including furniture and bedding.

To access Freecycle, simply search for a group in your area, join, and start browsing the listings. Keep in mind that availability can vary, so it’s worth checking back frequently and being quick to respond to any postings that match your needs. Remember to arrange safe pickup or delivery when communicating with other members.