When the pH will be depressed farther than desired, then adding a bit of alkalinity could be the best way. In that case, use baking soda or lime. Lime is the preferred option since no sodium is added. However, last week during our discussion for the upcoming Water book, we came to the conclusion that using baking soda MIGHT be OK as long as the Na concentration is kept below 50 ppm. If you had no sodium in your starting water, 0.5 gram of baking soda per gallon raises the sodium to 36 ppm and the alkalinity rises to 80 ppm. That might be enough for many brewing situations. If your water already has a lot of sodium, then this option is probably out.
I have used baking soda for my dark beers since I have low sodium in my well water and have yet to come across pickling lime in my travels. I also have some potassium bicarbonate laying around, but I haven't used it because none of the water calculators I've seen really address K+. I know potassium can give some saltiness similar to sodium, but I have no idea what the flavor threshold would be, or if there are any other adverse consequences to using it in the mash. Any thoughts?
You can find pickling lime at a farm supply store in the canning section. Or in season the big supermarkets will have canning supplies.
There is always online. http://www.fleetfarm.com/detail/mrs-wages-pickling-lime/0000000013296?utm_source=googleps&utm_medium=shopping%2Bsearch&utm_campaign=google%2Bproduct%20search&gclid=CI-kqsKU47UCFShgMgod0CwA7g