You don't need an alternator, you need...
You don't to take the car to a shop. This is a classic problem. All you need is a voltage regulator. It's about $40.00 for a Bosch unit and $20 for other aftermarket brands and takes about 5-10 minutes to install. Two small screws and it's out. I ran into this problem several times in MBs and BMWs. The alternator in these cars is SOLID and YOU DON"T NEED TO CHANGE IT unless it makes a bearing noise. As a matter of fact, Voltage Regulators are one of the best insurance items to put in your glove compartment if it was never changed and they usually fail at about 175K miles for Bosch units (from experience).
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer