Whole Foods gets a bad rap. They have these things at the bulk bins and things are reasonably priced.
I use enough vinegar to make the powder wet then add water to desired viscosity. Adjust along the way with water as it thickens on its own.
You'll never have mustard this good, you just won't. The vinegar causes the mustard granules to explode and it really is best when it is first made. All the hyperbolic effusing the folks in Dejon make over their mustard claiming their water is purest on Earth and their terroir cannot be duplicated is all Francocentric braggadocio. They import most their mustard seeds from the U.S. and from Canada, and the water we use is run through a ten stage filter.
One summer day a few years ago I took a bunch of homemade pretzels to one of my favorite local shops to share with the workers there. One clerk said, "All we need now is some mustard."
I reached into my backpack and whipped out a tiny jar such as this one and gave it to them. Much more mustard than needed for the pretzels so somebody got to take the rest home with them.
It really is better than anything that you can buy. Even if you use ordinary ground mustard from tins you get from the grocery store.
If you like mustard I recommend trying this. You won't be disappointed. Plus it's fun.