Look for superabsorbent balls whose expansion is reversible
Yes. The ones I have are known commercially as Jelly Marbles or Orbeez. They're sold at gardening stores, home decor, and online. They're used as toys but also as gardening products that absorb excess moisture in wet conditions and supply water in dry conditions. Mine grow up to about 1.5cm from a starting diameter of ~0.1cm. I've been told that some grow up to ~7.5cm; and they're advertised as increasing by 100-300 times in volume at full saturation. The process is reversible (e.g. by putting them in a cup with a wad of paper towels). I use them as a drying agent where powdered $\ce{CaCl2}$ etc. aren't ideal.

Note the actual identity of the polymer in the beads is a trade-secret, so I don't think you'll be able to find that information.