These are really easy to make - good instructions are here:
Neck Cooler instructions
My suggestion is to buy the best cotton abric you can find - go to a quilt store, don't buy fabric at Wal*** or Joann's - cheap, loosely woven fabric! Then preshrink the fabric (wash in hot water) before making these. This will keep the gel from oozing out through the fabric. Also since they are wet most of the time, you'll want to make sure the fabric is colorfast (washing in hot water will show you!).
Here are some sources for the polymer crystals, but you can get them at your local home center - they are the same crystals you add to potting soil to help retain moisture in the soil:
Medium Granules from Watersorb/Polymers Inc., (501) 623-9995,
www.watersorb.com.
Polymer Crystals from The Artistic Shop LLC, (262) 691-1353,
www.theartisticshop.com.
Soil Moist from JRM Chemical Inc., (800) 962-4010,
www.soilmoist.com.
Water Crystals from WaterCrystals.com, (719) 599-7141,
www.watercrystals.com.