Hi Melissa! The Disney water parks are so much fun, you can definitely make a vacation out of "water park only" trips!
You CAN buy tickets at a number of ticket dealers online. If you google "buy tickets to Disney water parks", there are many that will show up. I have personally never used any of them. I have friends that live near Orlando that have used the website www.orlandofuntickets.com before to buy passes to Disney and Universal Studios.
For official Disney vendors,
the website states that you can buy them at the water park itself, or at your resort. We decided to go to the water park one time when we didn't have the water park option on our ticket, so we hopped down to our resort front desk and bought them that morning. It was very simple. There are also many non-Disney area resorts that have official Disney World tickets available in their lobby area. We have also bought water park tickets at a non-Disney resort on one of the few trips that we stayed off property.
So, you can use your own judgement on whether to go ahead and buy or wait until you get there! I am sure it will be fine either way!
Let me know if you have any other questions as you plan!