Buy a tin of Coleman's mustard powder and make up you own!
As far as I remember the instructions are on the tin.
If they are not, try 1 tablespoon mustard, 2 tablespoons sugar, a pinch of salt, ditto pepper and as much vinegar and water as needed to make the consistency you want.
Taste it cautiously, and add more sugar, or honey, if it is too sharp.
You can use malt vinegar, or apple cider vinegar, as you please.
Alternatively, you could look in your shops for German mustard - there is no shortage of it here.