For spices that have a common flavor like mustard you are safest with staying with the more well known spice companies. McCormick and Colman's both are great spice companies and make dry mustard seasoning in small and large containers for your most convenient use.
I would think about using a brand that is more common then most brands because it has a good name attached and has been around from a long time. I used alot of dry mustard in my cooking from salads to dressings. I have had really good luck using Durkee Seasoning. They have been around for a very long time.