You might try looking for someone who manufactures ductwork or dust accumulators. We have had success in throwing oddball types of large sheetmetal projects with them. They would likely have to fabricate the spiral from sheet rather than try to bend tubing (likely to buckle or distort unless you have really long bend radii). I would look for someone who could laser or jet cut sheet stock as they could generate better curves for you with a programmed layout.
Go to "Services Offered" > "Roll Bending" > "rolling a helix". If they can't do it, they might lead you to someone who can. Also try:
