Is there a substitute for Dijon mustard and if so, what is it?
Dijon has a strong taste from the liquid they use in it (it's not quite vinegar, not quite wine).  I'd probably try a blend of some other mustard + a little white wine vinegar (not white vinegar; white wine vinegar) or champaigne vinegar or apple cider vinegar.  (Maybe even a dry white wine, if you have that on hand).

If you have dry mustard powder, and some time to let it sit, you could also try making your own; most of the recipes online seem to use a dry white wine as their base, a few use white wine vinegar.