Adding some good old sphagnum peatmoss will help get the soils more acidic. However you want to do a pH test to be sure where the soils pH level is first. Also adding some rabbit manure to the soils will bring the acid level up as well as provide some soil building nutrients to the soils. For a quick hit of acid if you are sure it is low, water the roses with some Miracid by Miracle Gro.