In all the years I have been shooting/building/buying guns and gun related stuff, I have never seen a 788 7-08 barrel for sale.
Your best bet is to buy a blank and have a gunsmith install it - there are a few in Wpg/Brandon area that do that work.
It isn't cheap tho. Ultimate Accuracy (Selkirk) has done a half dozen of my rifles and its a $600-800 touch for barrel, reaming, and installing.
Good luck!