The problem with replacing /individual/ cells is that, even if they're
exactly the same cell used in the original battery pack, the new cells will
have higher capacity, increasing the chance of cell reversal. You really
should replace all the cells.

My experience with Black & Decker's OEM battery packs -- if you can find
them -- is that they hold up well. I wouldn't purchase a third-party battery
pack, unless the supplier offers an iron-clad guarantee.

You might consider putting together a battery pack of 2500mAh or 2700mAh AA
NiMH cells. You'll get higher capacity, though the battery will take longer
to reach full charge.