Dave, what do you plan on making? As Kevin mentioned, turmeric is typically used for color however does add flavor. A little goes a long way, hence the recipe requesting only 1/2 tsp. Definitely do not substitute curry for turmeric, it will impart a complete different taste since curry is made with several spices.
It really depends on what you're making, mustard powder might not be a good substitute. It might be in your best interest to skip it all together as Kevin mentioned.
I have never seen fresh turmeric in the states, I wonder if it's available.
Erik