If your pigeons free range they will pick their own grit. Most propriety grits are similar with only slight variations. Grit / sand from the beach will have iodine, etc in it. Soil from mole hills will be full of nutrients also. Free range pigeons pick their grit from a range of places depending on their needs at the time. Find a grit from your local suppliers and add to it as you feel necessary especially if your pigeons are on a controlled system.