sounds like you're past this stage - Awnings By ZipDee
this may be helpful to you - Awnings By ZipDee
given its size, that tube will probably have to come to you via motor freight.
at a guess, and based on my own experience this past August as to the availability of a large Zip Dee part from a dealer, I'd otherwise suggest trying the factory directly, as I did.
otherwise, I'd tell you that I've had generally very good experiences with parts from Colonial Airstream in NJ, and Out of Doors Mart in NC.