...or go into a smoking lounge while in transit through a major airport and collect used cigarette butts from an ashtray or two.
Or go into a peep show...take a new hankie and wipe it all over the floor....then drop it at the scene of the crime...ewww
It goes without saying but, pick it up with tongs, place inside plastic ziplock bag, throw away the tongs.
Of course this would have the problem that several DNA profiles on the hankie would likely match known local felons...causing untold mischief and hilarity when they were all dragged in to explain. ;-)