What.is a good substitution for wasabi powder?

I agree with rt21...most "powdered wasabi" that is sold in the US is dehydrated horseradish and green food coloring. Because you have to reconstitute it to get the heat released, you can probably use prepared horseradish or if you happen to have a stalk of the fresh root (widely available at most markets now)...run a 1-2" piece over a micro plane or thru your cusinart shredder and it will give you a fine texture without as much "wetness" as the prepared horseradish

If its going into a dressing mustard powder will give you the heat or prepared horseradish if you have it