I ordered two Flashpoint (Adorama) batteries when I got my K-5 on the $989 fire sale. At a meager $9.95 each you can get 4 of them and still spend less than a replacement Pentax OEM. I charged up one of those and used it in the camera first (before the OEM) and its still going strong...
Flashpoint ADDLI90 Replacement Battery, Pentax Camera DLI90
For the record, the original Pentax battery is rated at 7.2v at 1850 mAh. While the images show the Flashpoint battery at 7.4v at 1500 mAh, this is incorrect. These batteries are actually 7.2v at 2000mAh! While you may be able to save a dollar or two ordering them on a place like eBay, Id prefer to spend the money at a place like B&H/Adorama, where their customer service will take care of me if there were an issue with one of the batteries.