Oddly enough, the 6V lead-acid batteries these things use are still widely available.
The charger, now...
I got this at least a decade ago and while I'm pretty sure I had a charger at one point, I don't seem to now. I say "Don't seem to" because although the charger receptacle on the battery box takes the same plug as do the desktop and handheld computer HPs (such as the HP-97), I kinda doubt that charger would work well. For one thing, the battery box notes:
"20V / 500MA MAX INPUT"
20V to charge a 6V battery seems high to me, but in any case it's way more than any of the chargers I have put out. Perhaps it's because the charger would be expected to run the disk drive (old-style Sony 3.5" drive and a large controller board) while the battery is charging.
I guess I could simply put the battery on a car battery charger set to 6V. That should work.