SnoWOW! Magic Snow Powder Just Add Water
$5 (3+2 ship each) for 20oz of sap that apparently expands to about 1.5 gallons. I think you'd better not use the whole pack in one diaper (BOOOM!)
That's about $4/lb. Polyacrylate goes on ebay for $16-20/lb plus shipping depending on how much you get, so if this is the same stuff then it's a better deal.
(I got in on an even better deal a little while ago at thatdeailydeal.com where they were selling the same thing for $3/20oz and flat rate $2 shipping, which was an even better deal - I called gearxs and they wouldn't budge)