It depends what is wrong with it.On modern alternators,everything is built in.The most common alternator faults are,worn out brushes,bearing or voltage regulator.Have it bench tested.If it is diagnosed with "Diode Ripple" it's more than likely the brushes causing it.All these parts are available,if you are savvy enough to take it apart and rebuild yourself.Be careful when taking it apart or re assembling it.General Motors alternator design,makes it hard to accept error.