I need to order a replacement battery for Model A1185. Where can I get it?

Get one from Apple.

There are third party replacement batteries available; they are not worth it.

You can get a new third party battery fairly cheap on eBay. Every MacBook from 2006 through Mid 2009 except the Late 2008 Aluminum Unibody and the Late 2009 and 2010 White Unibodies use the same A1185 battery.

Black Batteries http://shop.ebay.com/i.html?_nkw=macbook+black+battery&_sacat=0&LH_BIN=1&_sop=12 &_dmpt=Laptop_Batteries&_odkw=macbook+white+battery&_osacat=0&_trksid=p3286.c0. m 270.l1313

White Batteries http://shop.ebay.com/i.html?_nkw=macbook+white+battery&_sacat=0&LH_BIN=1&_sop=12 &_dmpt=Laptop_Batteries&_odkw=macbook+black+battery&_osacat=0&_trksid=p3286.c0. m 270.l1313

Or for $129 from The Apple Store http://store.apple.com/us/product/MA566LL/A?mco=MTY3ODQ5OTY

I've bought used MacBooks off of eBay for my family, nine of them so far over the last 6 years. I've bought 8 batteries off of eBay sellers, just going with the cheapest. Only had one with a problem, it gave out after 6 cycles. That one was replaced under the warranty by the seller. The only other batteries that have been a problem were Apple branded batteries that started bulging. I've had two of those happen.