Last year, I left my Canon 10D charger in the states accidentally when leaving on my field research in Japan. Rather than buying the $50 replacement charger from Canon, I bought a $10 clone charger/battery set from accproshop off ebay. It's worked great. The charger came with both a 120/240V wall-wart power supply and a 12V car DV lighter plug. The wall-wart makes it larger than the original Canon but the presence of a 12V plug is nice. Shipping was $8 to Japan.
To be on the safe side, I always unplug the charger after the green light turns on (don't want to fry the batteries). The $5 BP-511A replacements I got from the same place have also worked out great with my 10D. I've tested them out and they last just about as long as the genuine Canon ones, which cost about $60 each.
Accproshop is the store I used on ebay, but there are many sellers, mostly located in Hong Kong or China. Just search for 'BP511' or whatever battery type you use. There are clones of all of the major manufacturers. Prices range from $5-10 each. Watch out for shipping -- many companies charge $15 shipping/handling on a $5 battery -- so you can tell where they are making their money.
If you know of other sources, feel free to post them as comments to this thread.