Homemade mustards should be tightly sealed and stored in a cool, dark place. They will keep this way for months and will last indefinitely if stored in the refrigerator.
English dry mustard has a fiery bite. French dry mustard is a milder version. Choose accordingly when planning your shopping list.
Natural food stores and shops selling herbs and spices in bulk often have mustard in both dry powder and whole seed form. Look for both the yellow and black varieties of whole mustard seed. Each will impart its own character to the finished product.
Small jars and bottles can be purchased from cookware shops, hardware stores and chemical supply houses. Or you can save your own interesting jars. Wide-mouthed bottles and jars with clamp-type tops are preferable; they allow you to reseal the mustard jar after each use, and are handsome to use as well as to give. Make sure all jars are freshly washed and completely clean. Dry well before filling.