Source for Casco Silent Flex Ducts?

I am considering using the Casco Silent Flex II ducts for all runs that enter/exit my theater area. I called them today to find a distributor, but they told me that there aren't any in my area. I asked about buying direct, but they said they can't sell to a homeowner. Does anyone have a source for these ducts?

Also, Do I need to replace all of the ducts that are in the ceiling of the theater, or only the ducts that are open into the theater directly? My plan is to tape all of the seams of the rigid ducts with metallic tape, use dynamat to quiet "popping" noises and rattles and use Silent Flex to replace ducts that are open into the room. Is this the best way to stop sounds entering/exiting the theater from the ducts?

I was able to source the Casco Silent Flex II ducts for about the same price as the Home Depot non acoustical flex ducts (including shipping). This is for the 6" size. I saw that some people suggest stepping up to an 8" duct to reduce the sound of the air entering the theater. Does it make that much of a difference? Are there any other advantages to stepping up to the 8" duct? The price difference is about $7 per 25' duct, plus some additional shipping costs.

The advantage of the 6" for me (besides cost) is that I can curve them back and forth within the joist to help reduce the sound further. I'm trying to decide which duct will give me the bigger advantage (if any). Right now I am leaning towards the 6".