On the second question:
Millar Potter Boat Restoration
613 692 3455
and this guy... who may know more contacts than anyone
Both of them have done Shepherd resto's so they may have some ideas.
On the second question...
I may have spoken without conferring on exactly the size you need... If it is 2.5 inch OD then it's a little more difficult. That was a Canadian size mostly, and Alaskan Copper list it, but don't make it anymore, nor do they make 3.5 OD.
H&H Tube in Michigan will generally do those sizes, and since they are smaller, the minimums are manageable for them,and you.
Great Lakes Copper in Alabama and London Ontario may also make this size, but they don't sell to the public, only to wholesale.
Ansonia Copper and Brass do keep some of these sizes on hand, but as I said, I might have spoken too quickly with getting an exact measurement and wall thickness. They do a lot of different alloys too, including phosphor bronze and aluminum bronze. They're in Connecticut.
Continental Steel and tube in Florida do large size brass and copper as well, but again, I should have asked exact size.
McMaster Carr? They're just as surly to deal with as ever I see. Useless gits.