I have a nice cordless phone that I've had for 6 years but the batteries only last about a year. First I went to a regular store and paid almost as much as the phone cost for a replacement battery. The next time I checked Ebay and found hundreds of replacement batteries. After sorting through the ads I found an excellent Ebay store that cut my battery costs by 90%. The seller has excellent feedback, almost 100% and he sells a lot of items. I've bought my replacement batteries from the same seller ever since. He doesn't charge unreal shipping charges like a lot of sellers do either.
Some big things to watch out for: Feedback, if it's not close to 100% then go elsewhere or You'll probably get ripped off. Shipping charges: Some sellers sell these items straight from Japan and not only do You have to wait 2 weeks or more to get the item it'll cost You more for shipping than the item costs. Then if You get the wrong battery just try returning it to Japan, what a joke!
The most important thing is: SHIPPING COSTS DO COUNT TOWARD THE FINAL COST!