Distilled white vinegar has a pH of 2.4 and lemon juice has a pH of 2.2. This time of year you can buy dry pH down from Walmart or Lowes, look in the pool cleaning/spa section. Pretty cheap but I've never used it and don't know how to mix it.
If it's soil you are using, try mixing in dolomite lime next grow. It's a natural ph buffer. Then again you could also get hydroponic ph up and down - it is so concentrated i only end up adding one or two drops to a gallon of water/ferts to balance pH. Coffee grounds can also lower your pH - just use it as a top dressing.