Donate to a Pet Rescue Center
Humans aren't the only ones who can benefit from old mattresses. In addition to shelters for people in need, you may also want to check with local animal shelters. They could use your old mattress as beds for animals they've rescued who are awaiting adoption. A doggie daycare or other similar businesses might also be interested in your old mattress. Call around to see who might take it off your hands. Again, you'll need to have the mattress professionally cleaned before donating. However, an animal shelter might not be as concerned about a stain, as long as they know the mattress is clean. Ask if they will send someone to your home to pick it up in exchange for getting it for free.