Go on crutchfield.com. Go to "outfit my car" enter your cars year/model/etc and pick what speakers you like(look at reviews as well for an idea). Compare prices with other stores and decide where to buy from. If you are still running stock oem deck though, it might be about time to pick something else to replace it.
IMO, I woud go into best buy, find the speakers you want, then find them on AMAZON for the LOWEST PRICE. Happy Shopping!!!