you steal them from graveyards of course, or hire children to vandalise people's gardens, but only overgrown ones, where there's no evidence of any care, let alone pesticide... Or you buy organic ones yourself and plan for next year.

Or, if you are subtle and quick to anger, forget the rose thing and do it with crab-apples instead, which gives you a really nice clear green jelly looking fab with the skellies in it. Don't know the crabapple/sugar ratio though, and I suspect it's important: raw crabs taste like wasps sound.