It sounds to me like you want to do this organically... there are two options which are always mentioned in any article dealing with this - elemental sulfur and aluminum sulfate... HOWEVER, while elemental sulfur is the safest, it also takes 3-6 months of warm weather to work... aluminum sulfate is much faster acting BUT aluminum is toxic to plants...
Bite the bullet here and get some of General Hydroponic's PH-Down - depending on the pH of your water or nute solution, it usually only takes a few drops to lower the pH of the water and thereby the soil as you water/feed.
Good luck