My guess is that they probably get the pack from another supplier. I'm sure with a little digging, you could find something with the right voltage, capacity, layout, and connector. It may or may not be worth the trouble, though.
The other issue is that maybe the charger isn't working correctly and the batteries are okay. You may replace the pack and still be out of luck. Is it a Ni-Cad pack? How old is it? The life of the batteries can depend on several factors. Some batteries can form a "memory". On these, if you ran the batteries down to 50% everytime and then charged them up, after awhile, the battery will perform like it only has 50% capacity. Temperature, over-charging, how long it sits unused, etc. all play a factor into battery life. Also, with it being a "disposable" unit, I'm sure the quality of the original pack may not have been all that great either.
You may have one or more dead cells in the pack. In this case, the pack may appear to be charged, but putter out when a load is put on it.