There are several different "26-inch" bike tires available these days. To sort out the differences, a system of nomenclature was developed by the European Tire and Rim Technical Organization (ETRTRO) that is now known as the ISO (International Standards Organization) sizes. They are specified using the rim diameter in mm and the tire width in mm. So watch the ISO numbers carefully to tell what you need.
The most common 26" tires are Mountain Bike tires and they have an ISO rim diameter of 559mm so the tires will be marked 559 X Y mm where Y is the tire width. These are most likely what you got at Wal-Mart.
There is a 650C rim typically used on small frame road bikes and often referred to as a 26" wheel. It is ISO 571 mm.
Schwinn cruisers used a 26" wheel that has an ISO size of 571 mm and should be marked "S-7" on the tire sidewall. Schwinn lightweights used a 26" tire for an ISO 597 mm rim marked "S-6".
Along with these there are a few other "26-inch" rims and tires out there so be careful to match EXACTLY what you took off the bike. Good luck.