I purchased a pack at Michael's (they are located in the floral section if you're looking) and you can buy them in two types, already hydrated and ready to go, or dehydrated, which is what we did. I poured about a fourth of the package into a huge bowl and filled it with water one morning. By the time Bea was up from her nap they were ready to go.
This was by far her favorite sensory project we've done. She loved everything about them, the feel, the color, scooping them up and putting them into her bucket and then back into the bowl. I think we were outside playing with them for well over an hour. They have this crazy slippery gel like consistency, I think I enjoyed them as much as she did!
The only drawback is that they will break if you squish them too hard. So now we have bits of water beads all over our backyard, but hey, at least it's not in the house! I also need to check into storage. I think you can store them in a Ziploc (minus the water) and they will last a few weeks. We, of course, forgot them outside overnight and it was a little too cold for them. Oops! Or as Bea would say, "Whoop a see" (that's her version of whoops).
I also found this great idea for water beads over at Modern Parents Messy Kids for Easter. Since I'm not planning on giving her candy and chocolates this would make a great alternative.