Your grocery store will almost certainly be the most expensive place. You'll probably find alternative sources that are cheaper. It's just a loose leaf notebook that has one sheet for each frequently purchased item. While you're in the store make a note on the appropriate sheet when you've found a low price. Mark the date, store and price per unit.
Some sales are better than others.
When you've found a good deal stock up. If not, only buy the minimum necessary. Your grocery budget is a great place to find savings. Only housing takes up more of your income. And it's not easy to reduce your housing expenses without significantly changing your lifestyle.