Check out the Kitables link for a kit that comes with some supplies for creating this project.
For this project, you'll need approximately 10 feet of paracord, scissors, lighter, tape measure, hemostats, watch, and a 5/8" side release buckle. I used a 5/8" ITW Nexus contoured side release buckle, but you can use other less expensive ones like those found at Creative Designworks.
Paracord can be found at local Army/Navy stores, hunting/fishing outlets, and craft stores like Hobby Lobby and Michaels, and from many online sources including ebay and amazon...
The actual amount of paracord that will be used depends on your wrist size, cord, and tying technique(tight/loose). My wrist is about 8.5 inches and I actually used around 8 or 9 feet after finishing the bracelet/watchband. So using 10 feet is usually a safe estimate for most folks, since having too much cord is better than coming up short when making your project.
*If making a watchband, the watch needs to have about 5/8" space between the lugs(where the watch pins go) so that 5 strands of paracord will fit.