I picked up my dried rose petals in an international market and found them with the herbs and spices. You may also have luck finding them with the ingredients for herbal teas or even at a tea shop. Failing that, follow the link in the recipe to find them online. One other thing: if your rose petals come as buds, you’ll want to pull off the petals and discard the green base of the buds. Only the petals are edible.