Suppliers to the HVAC firms is one place.
I know Lindab is a company that makes them. I think JET sells them as well.
They're out there. If you go into many commercial/industrial establishments, it is what they use often for their forced air ducting or exaust.
I used 4 inch PVC for my DC collection system and I did not understand what was really meant by "Static Pressure" of plastic piping vs. metal piping. Thankfully I have a small shop with short runs, as I realize now that static pressure is like having the inside walls of the ducts wet insofar as what they could pull vs the same size in metal spiral duct.
The other advantage is there should not be grounding issues with metal pipe, whereas plastic, although they say that in a home hobbiest using less than 5HP DC there should be no concern, I don't want to be the one in a million to find out otherwise, so I grounded my piping inside and out.
The cost for me of the PVC vs Spiral was $600 vs $$750. I honestly decided on PVC for simplicity of implementation and the fact I could run down to Home Depot and get joints and connectors as I went along, rather than have to deal with returning unused components or acquiring additional components at greater expense and delay using spiral.
Here in Canada, the only source I found within reasonable distance of the grreater Toronto area were all wholesalers and have a list price for suckers and a contractors price which is 40 to 60% less if you can get chummy with them and get them to agree to sell it to you at that price. I actually did get one of them to agree to 50%-60% discount on the initial order.
Do your homework as to what you need and have a list of what you want to buy. Lee Valley has an excellent self cleaning metal blast gate that was cheaper than the non-self-cleaning ones sold anywhere else I found which is what I used for mine.
Good Luck...Kevin