ever hang out at a bar? know anyone that smokes? have friends who like to light fires for fun and recreation? ever light candles?
you probably know someone who uses a lighter for one reason or another. and most likely that lighter is a disposable butane lighter. there is a flint in every one of these lighters (not piezo electric) that is made to outlast the butane contained therein. why buy flints when you can reuse that spent lighter's flint in your zippo?
people routinely leave 'dead' lighters at bars and in trash cans all over the world. if you can find one or get a friend to give you one, you have all you need to complete this instructable. i assume you have a zippo or you wouldn't be checking out this particular project.