When I first got into boating, radiator shops would cut out tubes from the bundle and solder new ones in. With labor costs at what they are, that doesn't seem to be the case anymore. Seems like a big waste, but we've become a throw away society.
The last replacement I bought, I ordered on line from the "Manifold Warehouse" in Vancouver, BC.
http://www.manifoldwarehouse.com/mes/mgr?brch=home