I have seen a number of newly rebuilt alternators/starters/generators returned as cores as well. I contracted for a stint with a generally highly regarded remanufacturer. "remanufacture" process is much to be desired.

Process is as follows(at the location that I was associated with):

1. Remove cores from 55 gallon drums and teardown, then separate indiv. parts for cleaning.
2. Send parts through industrial cleaning machines of various design, depending on material.
3. Assemble "remanufactured" units with new bearings.
4. Send through "test bench" (means put unit in box)

Most all of the units reused brushes. Only the old Delco SI-10 was to get new brushes. Most popular units always got new bearings. Any less than popular units reused old bearings. Old bearings were sold to highest bidder for metal content.

Note: Units were put in boxes with more than one store/brand name.
Probably more than 1/3 of cores were clean units, 1/3 were rough junkyard leftovers.

If I have to buy a reman from somewhere, I have it tested before I even pay for it. I choose to repair myself or take to reputable electric shop- the one in my hometown largely was less expensive that the parts house (not always the quicky-part store, though).

Look for a company called National Automotive Charging. I worked eight years for a shop that was a dealer for their parts. I saw ONE alternator in that time had an early failure. Sadly it was my grandmother's car. not sure if they are still around. We carried their starters, water pumps, and starters.
Pricing was excellent. We could sell with good markup and be just over our cost from the parts house for the same application. Likely, if you find a dealer for them the guy at the shop will get you what you need for a reasonable price.