If you are unable to find it in a baking supply store, you can order it online. Look for gel coloring, as it’s concentrated. I use Americolor. I’m sure there are other brands as well. I prefer Americolor as I don’t find they give the food I use them in an “off” flavor. I’ve ordered from this company, here is a link to the product https://www.sweetbakingsupply.com/bright-white-americolor-food-color-p-237.html