Battery replacement help?

I have a Macbook Pro Unibody mid2009, 2.53, 15.4", 250gb (upgraded to 500gb) and I am having a heck of a time finding a replacement battery that meets the specs of my original?

My battery as recorded with Coconut Battery 2.6.6 show the original capacity was 6500mAh. Every replacement battery I see is a maximum of 5200mAh. Why would I buy a battery with less capacity?

Also, I am pretty sure that my model # is MB118LL/A if that helps, I am looking for the best alternative for a replacement and if that ends up being an Apple battery, then that is ok.

Hello,
The best choice would be an apple battery although they are a bit pricey.
The question is if you really want to put some unknown battery into your $$ machine and run the risk that it could damage something.

The number you listed is the order number, and I couldn't find a model number in MacTracker for the mid2009 MBP because that model is discontinued.
However, it should take the standard MBP Uni battery, I have the late 2008 MBP Uni and the battery is exactly the same one as shown on the apple site.

Genuine Apple Battery MBP Uni 15"
Rechargeable Battery - 15-inch MacBook Pro (aluminum) - Apple Store (U.S.)