The short answer is yes you can buy a new tube, but like Overlander says, you need to be concerned about the condition of the remaining hardware. Tube damage usually means overload on the canopy (water, snow, etc) which can damage all sorts of awning components.
The other issue (I'm dealing with at this moment) is replacement parts availablility. If the unit is old you may have to do some digging, bartering, or (in my case) modify newer components to work for your application.
"One of the best lessons I've learned is that you don't worry about criticism from people you wouldn't seek advice from."
William C. Swinney