Just make sure you use flowers that are meant to be edible and aren't doused in pesticides, and you're golden. I usually buy them pre-packaged in the herb section at Whole Foods or similar upscale supermarkets, but I've also seen them at the farmer's market, and if you have a green thumb (which I decidedly do NOT) you could even grow your own edible varieties. Not convinced it's as easy as I'm making it sound? Look, I have a Candied Flower photo tutorial right here. Told ya so.
Candied Flowers Photo c2011 Elizabeth LaBau, licensed to About.com, Inc.