Amazon has the MJSI HYR270 HydroRight Drop-in Dual Flush Converter for $19 with free shipping on $25+ purchase. Converts your toilet into a water-saving two-button version with a quick flush for liquids and paper. Approximate savings of 15,000 gallons per year for a family of four. Easy installation, but will not fit all toilets, read product description for details.
I bought one of these from Costco. It works very well after you adjust it to use the maximum amount of water it allows.
No free shipping on this deal, $25 with shipping.
The average price of water in the United States is about $1.50 for 1,000 gallons. So, this should pay for itself in about 1 year.
Somewhat mixed reviews. I'll stick with the low-tech "water saver" method: "If it's yellow, let it mellow; if it's brown, flush it down."
I prefer the primal tech version of shitting behind my house