^
Go to a craft supply store and find something called Water Crystals or Cracked Ice Crystals - they are used for flower arranging or other crafty stuff that crafty people do. They aren't solid crystals like the silica beads - these are the gel kind.
I got a packet for 99 cents that contains about a tablespoon of dry crystals. If you added water to the whole packet they would swell up to hold about a gallon of water (or so the packet says). You need like 1/10th of the packet to make enough of the hydrated crystals to fill a soup bowl. One "crystal" is about 1/16" in diameter when dry, but when wet it's about 1/2" in diameter (the ones I got are spherical).
I've used them for a while now and they hold 65% perfectly... cheapest, most stable humidification source I've found by far.
-JT