Cream of tartar is a fine white powder that is sold in the baking aisle in the spice/herbs/flavorings section of your supermarket. It is packaged in little plastic or glass bottles, just like the spices. You may also find it in bulk bags in some natural foods stores and specialty markets.

Online, it is available at quite a price in multiple small-quantity containers, or very affordably in small quantities in low-brow packaging, in large jars, or in bulk.