What to do
If a product is out of stock, find out whether the retailer has already designated a substitute item. Look for signs or ask customer service. If not, search the store for an equivalent product on your own and suggest that the store let you substitute that, just as I did.
Of course, make sure the substitute item is of at least equal value and that it's something you’ll be satisfied with. Check the package size, product colors and features, and, for substantial products, warranty coverage, Consumer Reports ratings, user reviews, and other details.
In 2010, our sister website, The Consumerist, featured an inside look at how Target employees were instructed to come up with substitute products for out-of-stock on-sale ones.
If you don’t think the item the store is willing to substitute is equivalent or better than the out-of-stock one, skip it. An alternative you often have is to obtain a rain check, which lets you buy the item you came for in the first place for the same sale price when it's available. In some states, including Florida and Massachusetts, the law requires retailers to issue rain checks for many types of sale items.
One caution: Retailers that routinely provide product by delivery, including online stores, may have terms and conditions that allow them to substitute an “equivalent” item automatically if the one you ordered is out of stock. If that doesn’t sound good to you (and it probably shouldn’t), check the fine print carefully before buying.