I usually buy them at either World of Disney or Marcelines. WOD now has the mobile checkout option. They are also sold at Bing Bongs Sweet Shop and Trolley Treats at DCA and at the Confectionery on Main Street. You might also see them in random gift shops as well as the hotel gift shops. I haven't had a hard time finding them. They sometimes put them on sale (all the bagged candy) when you buy 2. I load up when they do that.
They're not THAT sour. They aren't as sour as Sour Patch kids, for example. The reason I love them is they don't make my teeth sensitive like SPK do. I have a lot of gum recession and associated sensitivity that gets worse when I eat things that are too sour. These are just right in terms of sour level and my ability to eat a whole bag without issue.