Pickling lime is food-grade alum. It is used to firm up fruits with pectin during canning. There is no substitutes. In United States, Picking lime is available in canning supplies of almost all the food stores. I know Walmart carries it. If you have an Indian Store nearby, you can buy it there. In Indian stores it is called Fitkari. I am somewhat confused by your query. The only time you need Pickling lime is when you are making Mas from corn. Most of the American Stores (Walmart again sorry) sells Corn Mas flour that you can use to make Tortilla. I find using Corn Mas Flour much easier and simpler.
