Walmart is getting worse. Amazon.com and Ebay are shredding the walk in stores to pieces. Even the CVS and Rite Aids are losing it. Everything is getting purchased online. All the best diapers are online now. The adult diaper/adult baby stores do exist but, I have only heard of one in Atlanta GA. Try Googling abdl physical stores. Maybe there is one in some of the major cities.
As far as hoping for Walmart and the other stores to have what you need, they are only going to have the cheapest junk that will sell on a cheap and mass produced basis. The Internet shopping boom is part of the cause. You can get the best delivered to your door without having to leave the house. I believe the previous person's reply is right. You are going to have to have an extra case on hand out of pocket.