Dan, CBTN = Chevrolet by the Numbers, a series of books by Alan Colvin. They have a lot of great information in them.
The factory didn't use the same way of measuring cam specs as aftermarket sources. I don't remember exactly what the differences are, but John can tell you. I think it had something to do with where it's measured on the lifter rise (?) John???
"Where to buy" is another story and depends on what you want. You can get a "replacement" cam for the 396 at your local auto parts store! These are made by Melling or Clevite, etc... They work just fine for daily drivers as long as you don't go racing!! If you want aftermarket, then you can go with one of the "Muscle car" series of cams made by the various suppliers or go with a performance cam. It just depends on what you're looking for.