MuchMoreThanMuscle saidYou can't just expect to walk into a store and be accommodated at your own convenience. You also have to be a good customer.
If your size is popular/very common then either ask the manager when they receive shipments of new items (so you can show up and be one of the first to buy the item in your size) or simply show up more frequently to the store and increase your odds of finding something that suits you.
A store generally buys items in set quantities for retail. It would not behoove them to buy an excessive amount in a specific size and then wind up with too many that won't sell. This would mean they would have to have a clearance sale which means they lose out on money.
Just remember this:
Businesses do not exist to please the customer, businesses exist simply to make profit. The other option is to request that the store special order for you. But this would be at the store's discretion and they may deny you.
Perhaps I am being totally naive. If a size is popular/very common, is that not a very obvious guide to the retailer in deciding what sizes to order and in what quantities? The popular/very common sizes hardly ever end up in the clearance sales. The unpopular/uncommon sizes almost always do. I know retail stocking is not an exact science, but how is it that so many retailers appear to be bereft of common sense? How on earth in successful retailing can 'not pleasing the customer' and 'making a profit' be mutually exclusive?
You, the buying public, are a fickle bunch. Just when we think we have you figured out, you are on to the next thing. Most things you decide you want have been preplanned, and purchased months and months before they ever hit the shelves.
I work in buying for a large retail chain, and I can honestly tell you that although we collect mounds and mounds of data about buying habits, demographics and regionality of all items in the store, you can't get everything right. The lead time to get product in the stores in the quantities we need (and we're nowhere near Wal-Mart in size) is at least 9 months if not longer. We have to make buying decisions for a season before the previous one is even over. It's hard to be make accurate buying decisions, when you have one hand tied behind your back.
Trust me, the last thing I want is a pissed off customer because we don't have what they expect when they walk through the door. BUT, we also have to manage our inventories, minimize the need to clearance the stuff that doesn't sell, and maximizing the profit for the company AND the shareholders (they're the ones who ultimately decide if we stay in business or not).
Thus endeth the Retail 101 lesson . . .