I remember there was a thread about where to get Dr. Pepper. Couldn’t find that thread though.
Anyhow, Dr. Pepper can be found at Jason’s supermarket @ Taipei 101, same shelf where all the other canned drinks are located.
Note however that it seems to sell out rather quickly, went in on Sunday afternoon to look around and a few hours later, when I did some shopping, none was left.
If you can’t find it ask the staff for “Dr. Paper”. Well, that’s how they label it at least …