Amazon has the MJSI HYR270 HydroRight Drop-in Dual Flush Converter for
$17 with free shipping on orders $25 or more. Otherwise shipping is $6. 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 these, if you don't install it just right it will leak - kind of tricky and took some tweeking on two out of three of my toilets. My water bill was actually 10 dollars higher which made me figure something wasn't. Once I got it dialed in my bill did definitely drop