We always buy our Jibbitz online at
www.jibbitz.com
We like viewing the selection/assortment there and as another poster said, they now give you the option to design online. We combined some jewels, flowers, stars, and disney princess jibbitz to create a really cute set of "princess crocs"
You should be able to find jibbitz at every resort gift shop, select gift shops in the park (think "bigger" like Mouse Gears, the Emporium, etc) and also the "croc carts" scattered around the parks.
We didn't purchase any directly at Disney but I don't recall the price being to much higher than what we've seen online. I will say that when we have gone in specility shops around here to buy them, they were more expensive than what I've seen online; not sure if thats the case with Disney or not.
Some locations had a good variety of charms while others didn't. Jibbitz have exteneded their Disney-themed line and most of which isn't always displayed in the parks. We've also noticed a view "park specific" jibbitz that we can't find online (like cinderella's castle).
We found a new way to incorporate Jibbitz into our trips though. Each day we have a letter waiting for the kids to read when they get up welcoming them to a park/character meal/event/etc. In the letter was also a themed jibbitz to go along with the day's events that they could place on their crocs. It was so nice hearing the kids talk about the trip and pointing to each jibbitz and the story that went along with it!