Portable Mac Air Charger for flight

I going away on a 21 hour flight to Italy and all of my entertainment is on my laptop. My laptop defiantly won't last that long, is there a power plug on my seat on the economy seats on most planes or not? If not, is there such thing as a portable laptop charger for a Mac Air? i know there is a phone portable charger. If there is one where abouts can you buy one? I live in Australia so not much help if it sells at a store overseas.

Thanks :)