We just returned from Orlando, and yes, the glow cubes are available from Sci-Fi. I think they come with some of the drinks - others you can add them for a cost.
As for money saving tips - we always bring our own water bottle into the Disney parks and refill as necessary throughout the day. Some people don't like the taste of Florida water (swampy), but personally we don't have a big problem with it. You can always get those single serving Crystal Light pouches (or Lipton has iced tea ones now I believe) to add to the water.
I'm unsure about Disney, but I do know if you are at Universal Studios, you can buy one of their plastic glasses/mugs, and refills are $1.50. They are huge too. We usually stick to water at Disney as there are water fountains everywhere whereas at USF, they're few and far between.