I always knock on the door, say hello and ask. I've met some really nice people that way, and if they give me cuttings they usually have advice on how to propagate them

We have dozens of beautiful old rose bushes outside our office building, and one day they were all beheaded. We went hunting around for the culprit, and found out a woman wanted rose petals for her sister's wedding that weekend so took every last petal from the work garden