Note, thet you can also use "N" type accumulators instead of the batteries. This is more convenient than the accu pack. You must load them externally though.
"N" type accus also are larger in size (longer) than their counterparts in the accu pack (the accu pack includes the charger electronics - very primitive), so that the "N"accus will hold more charge than the pack.
HP18C, 28C/S and 19B/BII users: please remind that you can't use these accumulators (see the manual).
In these calculators the batteries last forever (maybe), so that there is not much need to use accus.
If somebody wants to get these accus, I'd recommend asking radio shack (don't know if they have them - but anybody says "ask radio shack", when somewone needs electronics parts). In Austria or Germany I'd use Conrad Elektronik or RS Components (I'm not affiliated with any of these Companies, others may have these accus too).