One of the first things you should do before buying a headstone is to check with the cemetery to see what their regulations are. Some require headstones of specific material. For instance, some will allow only "Granite-Bronze" headstones. These are bronze cast gravemarkers attached to granite and are quite expensive.
For the convenience of the cemeteries, some require only flat headstones set in a cement pad, flush with the ground.
Others might require over-sized stones with no cement or a specific shape & size.
Some cemeteries even have height limitations on upright headstones. Be sure to check with them so that you don't purchase something they will not let you use.