Great question! If your local store like CVS, Walgreens, Rite Aid, or Target is out of an advertised item, most stores will offer rainchecks on these sale items. Sometimes, certain advertised sale items may be excluded from rainchecks because they are while supplies last For example, sometimes Target will state "no rain checks will be provided - while supplies last" on particular items. However, in most other instances, these stores will typically provide rainchecks, so that you can grab the deal later when the items are back in-stock.
All you have to do is bring the weekly ad or advertised sale price to the cashier or customer service, and they will write a raincheck for you. Then, all you have to do is bring the raincheck in when the product is back on the shelves, and you'll be able to snag the item for the advertised sale price. Keep in mind that some stores' rainchecks will expire within a certain time period.
CVS, on the other hand, gives out rainchecks for out-of-stock advertised items, and they NEVER expire. Yep, CVS rainchecks last as long as you want! Just redeem the raincheck whenever the item is back in-stock, and you're ready to purchase the item! I seriously have CVS rainchecks in my purse from years ago because I haven't redeemed them yet ;)
If you haven't gotten a raincheck from a store before, it never hurts to ask! Most stores are required to provide rainchecks by law. It never hurts to ask if your store will provide a raincheck!