I assume you mean you can only find more 8-cell units. What's the actual rating on your current battery, or the model number? The mAh rating is what determines how long it will last. It's possible for an 8-cell battery to have more or less capacity than another 8-cell unit I think, but the ones I see all look exactly the same.
How old is your current battery? A new 8-cell might last a little longer, but even HP says 2h39m for DVD watching. If HP doesn't even carry an extended-capacity battery, it's probably going to be impossible to find a 3rd party one. You might just have to buy a few extras of this one.
Since you don't mind carrying an extra thing around, you might consider the "external battery" products. They're just a big battery with an adapter to plug into the AC port of your notebook. Horribly inefficient, but they might give you the capacity you need.
This unit is 9000mAh at 16V. Your HP seems to be 18.5V, you can look at the charger cable to see, so about 7800mAh, a little less than twice your current battery capacity. It seems there are few even of these external units with the kind of capacity you're looking at. You just have a very power hungry laptop.
