I always take my own, since I drive. I drink one each morning like other people drink coffee. I can drink water the rest of the day, but have to have that Dr. Pepper to function. I actually prefer the one made with sugar instead of corn syrup, so that really requires planning ahead.
If you can stomach Mr. Pibb, you can get it at the Satu'li Canteen in Animal Kingdom! I was kind of shocked to see that they served it there, as we didn't see Dr Pepper or Pibb anywhere else in the parks (and I think I maybe had it once somewhere in Disney Springs). I guess the Na'vi like Mr. Pibb?!
Otherwise, I go without. Its a pain having to lug Dr. Pepper in my luggage, TSA freaks out if you try to bring cans or bottles or soda in your carry on, and if I'm staying on property I'm not renting a car or taxi to go to an offsite grocery store, so I just treat it as I'm on a soda restricted diet when I'm vacationing at Disney.
That's too bad-when I googled "Coke freestyle machines" they showed Dr Pepper as an option so I was excited. In our local supermarkets I've seen the Coke person bringing in Dr Pepper along with Coke products (no Pepsi).