Texas Toast may be something you can use, although I'm not sure of its availability everywhere. It can be bought in the bread aisle at the grocery store where I live here in Georgia. It's basically just thick sliced white bread. It's about double the thickness of normal white bread. A lot of country restaurants around here will serve it buttered as the bread item of an entrée.
I used to work at a steakhouse when I lived in Louisiana and they made their croutons from normal white sandwich bread. For whatever reason, I remember specifically that they used Evangeline Maid brand sandwich bread, which is sold widely in South Louisiana. Those croutons were quite good, too.