For the most durable caber, you want a natural log that has its own taper.

You can taper a piece of lumber, but cut lumber is not going to be as strong and resistant to cracking as a whole tree. Cabers take a LOT of stress when they get a bad turn.

You can also use lumber without a taper and that actually makes a great practice caber. Without a caber it's "dead stick", and that much more difficult to turn. It does make it easier to pick, but harder to turn.