There's no simple answer to this becuase it will depend on your laptop, but have a look here:
http://www.mdsbattery.co.uk/shop/pro...ctGroupID=1244
and
http://www.simply.co.uk/productinformation/68629/WW/APC_TRAVELPOWER_CASE_(SYNTHECTIC_LEATHER)/
The one from Simply is a good set, I can't vouch for the first one.
Also you need to make sure that the airline AND aircraft you're travelling on have in-seat power. Most airlines only put this feature in Business and First classes (Premium Economy if you're lucky) and even if the airline does offer it many older interiors haven't been upgraded to include it.
You can also buy them on-board, usually about 100 quid. Some airlines will also offer them for rental for the duration of your flight - again check with your carrier.