The first thing you need is a tree! You can use a stump from a fallen or cut down tree or even look on craigslist or Next-door for help finding one. Once you have a tree you need to find a sawmill to slice it.
Did you know that when you slice a tree into round discs they are called cookies? I didn't until I started researching this project.
Once you find a sawmill to cut your stump give them the measurements of how thick you want your slices.