Wood stair parts are available at major home improvement stores such as Lowes and Home Depot. Local hardware and lumber supply stores also carry parts but selection can be more limited than at a larger store.
Wood stair parts are available at online retailers such as directstairparts.com and Amazon.com. Habitat For Humanity Home Stores sometimes have left over wood stair parts for sale. Craigslist.org is a good resource for left over or used wood stair parts.
Specialty stair contractors sometimes sell new and replacement parts to home and business owners. Local sawmills often make wood stair parts to specifications. Woodstairs.com carries a large selection of wood stair parts in a variety of finishes and color schemes.