Dollar Tree & Target's Dollar spot are about as cheap as it gets, unless you catch a sale.
Micheals--cut those 40% coupons out every week & use them.
Hobby Lobby--about once a month has 50% off most scrapbooking items.
You will be hard pressed to find cheaper online, though you could get lucky on ebay--you may have to spend more upfront, but you may be able to get a "lot" of stickers for a lower per page price, BUT you may have to sacrifice getting exactly what you want doing that.