If you can't find a good sourdough bread you can easily substitute a variety of other breads in its place. Try using any crusty bread cut into thick pieces. Breads like ciabatta breads, French breads or country loaves will work well. I also like using toasted rye bread or pumpernickel for a twist on the classic. In a pinch, you could even grab a bagel and make an open face sandwich out of it.