You can order the drink coupons 4/21.25 on Carnival.com . Here is the link:
http://www.carnival.com/BonVoyage/AddToCart.aspx?cat=MostPopularItems&pid=BAR
Also, as stated above, you can also buy a drink of the day card from the bar waitstaff, but someone who just got off the ship stated it is now $26 + gratuity.
The soda card also can be purchased from the bar waitstaff and runs around $27/children and $42/adults. It's called the "Fountain Fun Card". Here is the link for information:
http://www.carnival.com/CMS/FAQs/Fountain_Fun_Card.aspx
A one time 15% gratuity is added at purchase. I've been told that if you wait to purchase this card till after the ship is said, you will save the tax charged while still in Galveston. Also, water,lemonade,tea, and juice are served free of charge in the buffet area of the Lido deck.