Toto brand fill valves can be purchased either in-store or online at The Home Depot and online at Amazon as of 2015. Lowe's and Ace Hardware have Toto-compatible fill valves online and in-store, including models made by Korky and Fluidmaster.
The Home Depot, Lowe's and Ace Hardware all provide store personnel who can explain toilet fill valve replacement. The packaging on the boxes of the different brands usually has instructions outlining the simple maintenance procedure. Brochures inside the packaging contain more detailed instructions, such as a list of specific tools required and how to adjust the valves for optimum performance and minimum water consumption.