The Bosch alternators seem to do pretty well over the long haul, and a lot of Bricksters just find used ones in boneyards for cheap. With just one vehicle at my disposal I prefer a less chancy route. Coincidentally, the alternator on my '92 245 is making bad bearing noises at 210K miles, and I just bought a rebuilt unit in eBay (from a place with a decent reputation) for about $85, delivered.

You didn't state what year 240 you have, but if you visit the FCP Groton website you can figure out which alternator is right for your car. The part numbers shown are the correct for Bosch rebuilts, so you can find the right number and then shop on line.

Another route you can go is to talk to a couple of local parts houses if you know/trust them, and get a recommendation for a local rebuilder. I did this for a couple of my cars over the years, and I had my own unit rebuilt, so at least I knew its pedigree (e.g., it hadn't ever been immersed in salt water).

Lastly, if your alternator is a Bosch unit you should be able to buy a replacement brush pack for it for under $20. The most common cause of low output on high-mileage alternators is worn brushes. Replacing just the brushes is the lowest-cost choice.
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Thank goodness we don't get all the government we pay for. -- Will Rogers