I bought this one off amazon:
Amazon.com: 2800mAh Mobile Battery For LS670, Optimus S Extended With Back Cover: MP3 Players & Accessories
I ordered it 9/30/11 and received it today 10/11/11
It was supposed to get here in 3-5 days, but that didn't happen. I'm charging the battery now and then doing a calibration so we'll see how long it lasts.
EDIT:
The price seems to have gone up since I bought it. I only paid $20 shipped for mine. I bought it because I didn't want to wait forever to get it, but I ended up waiting a long time anyway. If I were to do it again, I'd buy the one listed originally in this tread.