Hair extensions????????? Help??????!!!!!?

Ok so it's almost prom and I wanna get some hair extensions to make my hair appear longer I need black or brown ones dark brown and like I want them to be 18 inches long and yeah can u tell me where to get them???? I live in arroyo grande zip: 93420 and I don't want them to be to pricy because their only for one day and I don't care if their fake or real but give me the name of the store and how much hey cost Plz and thank yew :)