Where i could i buy an older macbook air?

i am looking to buy a macbook air and before i go out buying the the newest model i thought i would try and get one of the 2008 models but the problem is my dad doesn't want me buying one off eBay unless its certified and in good condition. i also have no idea were to buy one i looked on eBay criagslist and the apple store. does anyone know were i could purchase a 2008 macbook air? thanks.