Take the original can of paint for that wall, if you have it, to the paint store of your choice. They can use the formula from the can to match the paint for you.
Find a small, out-of-the-way area, such as a corner hidden by furniture or the inside of a closet. Cut a small piece of the drywall with a sharp knife. Cut it about 2 inches square. Do not cut very deep, since all you need is the color sample. Take that to your local store and they can use it to match the color for you.
Purchase higher-quality paint. Lower-quality paint may be cheaper in the short term, but it is difficult to get the most accurate match with low-quality paint.