I was looking for a spare battery to keep me powered up whilst on a long flight to Oz - The best deal was to be found at HP! A suprise for me! They have 40% off at the moment. Not sure how long for but needeless to say I have bagged myself one. This is for a six cell one too!
I know it is more expensive that some China made ones on ebay but this is the real deal. No invalidating warranties!