Hi,
If you need to lower the soil ph and want to use something mixed in that'll break down over time you can add more of the peat(or other organic matter like mulch), otherwise any type of sulfur product will do the trick, if you have a garden store nearby just go ask for a soil acidifier or sulfur for your soil, i know most u.s.a stores carry "Epsomas Soil Acidifier"
Good luck with the problem solving. ✌️