Most chain stores (woolworths, Kmart etc) will give "Rain Checks" on advertised specials. This means that if an advertised special is sold out when you go to buy it, the store will give you a voucher allowing you to get the item at the special price when new stock arrives. These vouchers are often valid for several weeks. This is great for popular big ticket supermarket items (thinking nappies, washing powder, soft drinks) - you can often buy some for now and then go back at the end of the sale period when they are out of stock and grab a rain check voucher so that you can get some at the special price later.
Sometimes a Rain check is not available (could be end of stock line or super special price), but it's worth asking. Vouchers are issued at the service counters.