Some parts stores such as AutoZone will test your charging system in the car at no cost to you. This diagnosis often saves time and money.
I replace the voltage regulator / brush holder assembly first to see if it corrects the problem. On many vehicles it can be done by removing two screws with the alternator still on the car. The part cost is much less and the labor is about five minutes if access is easy. My best luck has been with Bosch regulators. Others may cost less but often have a shorter life or allow voltage surging.
If your alternator has over 100,000 miles on it, or is noisy, you may want to have the bearings / bushing replaced which requires removal of the assembly.