You can purchase soda coupons on Carnival's website. You need to be registered with mycarnival and have your booking number and room number, otherwise you need to call the 1-800 number they have on the page to order.
Once on board you can purchase a soda card which allows you unlimited fountain soda for the length of your cruise. It is $30.00 for a 7 day cruise for those over 21, $20.00 for those under 21.
You can buy the coupons at any of the bars on the ship. Cost is $30.00 for an adult. But unless you drink LOTS of cokes...you may come out better buying as you go. They sell the larger plastic bottles for $2.50 and when you are on shore excursions you aren't using is of course.
There is always lemonade and fruit punch along w/ coffee and tea on board 24 hours a day.
I brought a six pack of caffiene free Dr. Pepper in my suitcase since I knew they'd not have that. I did buy a drink card for my teenager for $20.00 and she ended up buying more cokes for her new friends she made on board, than for herself.