Buying a new drill/battery/charger is almost always a better choice. That's
assuming that you don't have a fairly modern, high-voltage, top-of-the-line
drill. Purchasing an individual replacement battery is expensive.

On the low end, you can pick up a drill/battery/charger combo at reasonable
voltage under $20 from Harbor Freight, Homier, etc. Even if you have to order
on-line and pay shipping, it is economical. The batteries will usually have
less capacity then higher cost units - this is due to intentional slight
undersizing or to lost cell capacity do to the age of the units. Not a big
problem. I have friends who have walked into Harbor Freight and bought 6 or 8
cheap drills because they cost just a few dollars more than the cost of the
battery packs. This way they've got extra drill bodies and extra chargers.

I've got a very comprehesive Black and Decker 14.4 volt setup with many tools,
batteries and chargers. I've watched for great sales and closeouts on this
system. For example, I bought one extra case, drill, light, charger and two
batteries for much less than the list price for just the 2 batteries.

As for your existing drill and battery: It usually isn't that difficult to cut
open the existing pack and replace the cells inside. Sometimes you may need a
dremmel tool to fit generic replacement cells inside the case if the
manufacturer purposely used non-standard sizes to prevent this. Sears is one
culprit for such games. The same advise goes for cordless phone battery
packs - often one needs to "Dremmel out" bits of the handset interior to fit
in replacement cells which are slightly larger than the intentionally sub-sized
originals. Any battery pack can be replaced - it's just a matter of how
difficult the manufacturer intentionally made the task, how handy your are, and
the value of your time.

I hope this helps.