If you have not already done this, check out John Palmer "How To Brew" chapter 15 on water and mash pH.
He suggests calcium sulfate and calcium chloride to lower pH and carbonates (baking soda) to raise pH. I use the baking soda as my mash pH tends to be too low.
From the comments here, the Five Star product seems to work well. A quick search shows it to be about $12 for a pound and you only need a tablespoon per 5 gallons. I have not used it but, I am considering trying some.
If you are going to add acid to your mash, make sure you use an organic acid that yeast can metabolize like acetic, lactic, citric, etc. Also, depending on what type of beer you are making, the pH of the mash will vary. Darker malts are more acidic so you may not need to adjust as much.
I hope this helps.