I bought a new battery for my 13" Black Macbook yesterday from BattDepot. The Apple replacement part goes for $159, I've seen third party replacements in stores for $129, but BattDepot lists theirs for $47.99 plus shipping and handling. After taxes, it comes to just under $60, almost $100 less than Apple's suggested retail price.
It seems that BattDepot is a reseller for Dr. Battery, so when I selected the "Pick up from warehouse" option ($5) I was given the address of the Richmond warehouse of Dr. Battery. The battery fits nicely, and is a good though not perfect match for color and texture. It is reported by System Profiler as a 5125 mAh battery at 12566 mV, once fully charged.
I think I'll pick up a new battery for my MacBook Pro next - I probably should have bought both at once and saved the extra $5 pickup charge. This might also be a consideration if you can organize a group purchase.
I'll report further if I notice any issues with the battery.