How can i purchase halo 3???

halo 3 is gona be realeased 4 days from now...(yay) but i didint reserve it..( awww) so if i go to the stores where is gona be selled at will they have copys in stock for the ones who didint reserve or only people that reserved are gona be able to get it???? and secondley if i reserve tommorow will i get my copy on sep 25 and... can i still reserve it in a store???