The best substitute for dry mustard in a recipe is regular mustard.
Dry mustard is just ground up mustard seeds that are used to make the regular mustard that you buy in a bottle or jar.
If you do not have any dry mustard which is mustard powder then use some regular mustard in the recipe which is the best thing to use in place of dry mustard.
When you use regular yellow mustard in place of dry mustard the recipe will turn out just fine.
I have used yellow mustard in place of dry mustard in recipes that called for dry mustard and the recipe turned out just fine.
Dijon Mustard or Spicy Brown Mustard also works great as a substitute for dry mustard in most recipes.