I don't really know anything about this but I can give you an educated guess and maybe get you moving in the right direction.
judging from what eeuroparts has for sale (FCP and IPD both are not selling this part for some reason) there is a 120 amp version and a 140 amp version, and there is one that comes 'with freewheel' and one that comes 'without freewheel'.
http://www.eeuroparts.com/Main/PartDetail.aspx?id=AL0756X
http://www.eeuroparts.com/Main/PartDetail.aspx?id=AL0805X
http://www.eeuroparts.com/Main/PartDetail.aspx?id=AL0820X
I suspect the other 'regular' alternator would fit just fine and even work just fine, but you'd see your fuel mileage decrease.
You might be able to order what you want online from eeuroparts or someone else, though I suspect shipping on something as heavy as an alternator would be quite high - plus you'd need to ship the old one back for the core charge.
The other option you might want to consider is a rebuilder. I suspect if you've taken it to a dealer they have not told you what is wrong with the alternator, just that it's broken. A real starters and alternators shop will be able to test the individual parts and tell you what's wrong, and rebuild it as long as there is no mechanical damage. And I'm sure their services would be much less than what the dealer is trying to charge you for a new one. A new voltage regulator is usually on the order of ~$100, for instance.
I assume you can't move the car without the alternator and you don't want to tow it, so maybe ask the dealer to remove the alternator so you can take it to a rebuilder? Have it fixed and then let the dealer put it back in for you? (I'm assuming you're not planning on doing the alternator install yourself? You should not be scared of it if you're a decent wrencher - it's not a terribly tough job, though a bit long.)
Regardless, I'm almost certain a rebuilder can give you a better-than-new alternator for less money.
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1998 V70 AWD->FWD Turbo 200k+
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