I've found that you can sometimes talk to the manager of the store and they can actually order most any product that any of their suppliers stock - or more of a certain product.

Our store would continually run out of the particular baby food we wanted and the cashier noticed me buying it all when it was in stock and asked if I wanted them to just order another box for me.

This doesn't help when the suppliers have discontinued it, and some of the larger chains are all automated, but it can be a useful trick. The computers try to notice what sells and order more (I've bought all of a certain item and noticed they stocked twice as much the next week, for example) but sometimes it can be assisted.

You can also email the company/distributor and ask for retail locations in your area, they'll usually know.