buy from the laptop company's distributor or straight from their websites. I've tried different battery stores and the batteries they sent did work like they said for the first few months and then either died or lost its holding power drastically just after 3-4 months. The uncertain Voltage fluctuation on those unknown batteries sometimes can damage your device.

Also, if you want to go into more technical detail, higher capacity batteries SOMETIMES need different amp. charger to efficiently distribute the power inside all the partition of the cells. So my suggestion is not to alter what your laptop company's suggest.

If you don't trust me, you can just go buy 4 regular rechargeable AA NiMH battery. Then get one regular charger (3-6 hours) and another fast charger (30minutes). Charge 2 battery with reg. charger and 2 with fast charger. Then use them seperately with 2 device (flash light etc). You can see that the batteries charged with normal charger will last much longer than the one charged with fast charger. The ones those charged with fast charger will have dead partition cell faster than the ones charged with normal charger in the long run hence shorter life span.