If you are looking for a free mannequin – try craigslist or freecycle. Forget about calling a retail chain and asking to borrow a mannequin from them. Unless you have a close relative or friend who works in the visuals department of the store, they just don’t loan out mannequins. If a retail store is going out of business in your area, they might let you have a mannequin then, but usually they want to sell them to you.
Should you need to borrow a mannequin, there are many places who will rent a mannequin to you. Not the same as free, but not as expensive as buying a mannequin if you just need one for a day or two. Go online and do a search for “mannequins for rent” in your city. If nothing comes up in your city, try places that sell used store retail store fixtures or prop rental companies.
However if you are doing an art project, going to Burning Man or doing a Halloween Haunted House display, renting a mannequin might not be an option, because you need one that you can alter or destroy. This is another time where you should do a search online for used mannequins and ask the company if they have a bargain bin with distressed mannequins.
Often distressed mannequins not posted on a website because the inventory changes too much. Or since the mannequins are slightly distressed they prefer that people come into the store to buy them instead offering them online, so that people can come in an inspect the damage for themselves. Once again, this is not the same as free, but at least you won’t be paying full price for a mannequin.