My grandma was terrific at sewing stuff recycled from older stuff. And she would take zippers from something worn out like old jeans and use the zipper in something else new that she was sewing. Nothing was ever wasted.
But old blue jeans with the zipper intact are worth more to some folks than just the zipper alone. Try consignment stores that sell used clothing - they may be interested in your old blue jeans - with zipper.