pH down is pretty cheap stuff - or just buy direct from a chemical supply store. Use a few drops until your watering water is maybe a bit under your target (say watering with pH 5.8-6.0 ) and water with that, monitoring your soil to see how it changes. Use nitric acid during veg (extra nitrogen) and phosphoric acid (extra phosphorus) during flower. How much you need will depend on what's in your soil and how strongly buffering it is. It can take a little or a lot to change soil pH, but go slowly and you should see results.
If you're against using pH down in the water, then, as said, adding some peat or compost to the soil can help, as can old coffee grinds, slices of apple, etc. If they're already well rooted, though, adding stuff to the soil can be tricky. pH down is really the simplest solution, I think, as adding new components to the topsoil will take a long time to diffuse into the mix.