Can i use an alternative to wire up a car amp?

ok i have a emerson mobile amp that has 600 watt rms. i think. and in the manual it says to use no less then 8 gauge wire. now i was wondering if it could be safe to use something like an old extention cord or va cleaner cord instead?!?! so yea. im basicly trying to save as mutch money as possible being that i am only 16. thanks