FASTENAL has both aluminum and steel square tubing.
http://www.fastenal.com/web/search/prod ... ubing%22|~
80/20 has tubing in wide variety, makes up excellent frames of all sorts, boxes, computer panels, I even used some to make precision fences for my table saw and bandsaw. The fittings they sell allow easy fabrication of most ANYTHING. Kinda pricey, in some respects...
WW Grainger carries steel, aluminum and stainless steel materials
http://www.grainger.com/Grainger/raw-ma ... alog/N-bis
And you might find a used awning at car wrecking yards, I've seen them quite frequently at the larger wrecking yards.
Contact insurance adjusters in your area, they might know of an RV that got totaled, parts might be available.
Likewise body repair and towing places.