Yes, there are stores that specialize in unfinished furniture in Knoxville,TN. You can find them at http://www.superpages.com/yellowpages/C-Unfinished+Furniture/S-TN/T-Knoxville/
Unfinished bookcases can be purchased in a variety of places. One could find them at home improvement stores such as Lowe's, Rona, or Home Depot or at specialty furniture stores that sell furniture in the raw or unfinished furniture.
There are a number of online stores that sell unfinished furniture. The Wayside Workshop, which is based out of Aurora, offers unfinished furniture. Woodcraft Furniture, in Beechmont, also sells unfinished furniture.
Headboards from any number of countries can be found at most of the finer stores, and possibly any furniture stores or stores that specialize in bedroom furniture.
You can usually find furniture at speciality stores that specialize in restaurant furniture. If your looking for quality, it can be somewhat of an investment.
You can find replacement bookshelf pins for your furniture at hardware stores, home improvement stores, or online retailers that specialize in furniture parts and accessories.
There does not seem to be good furniture store in San Marino that sells unfinished furniture exclusively, but a store www.millstores.com does deliver to San Marino. The normal furniture stores might have some unfinished furniture as well, but you might not get the extensive variety you are looking for. A good idea would be to rather travel some distance to a full-fledged store (check the one closest to you on the website suggested).
Unfinished wood furniture items can be bought on internet sites such as eBay, or from the private blogs or webpages of skilled craftsmen. Etsy is an online store where many craftspeople work, and one could easily find such an item there.
The "furniture" category will have plenty of furniture stores, some of which may specialize in children's furniture.
You can find replacement dresser mirror brackets at hardware stores, furniture stores, or online retailers that specialize in furniture parts and accessories.
You can find replacement vinyl chair strapping for your outdoor furniture at home improvement stores, outdoor furniture stores, or online retailers that specialize in outdoor furniture parts and accessories.
You can find office chair replacement casters at office supply stores, furniture stores, or online retailers that specialize in office furniture accessories.
You can purchase unfinished cabinets and furniture online at websites such as Mill Stores and Kitchen Cabinet Depot. You may also be able to locate a nearby wood store that sells unfinished cabinets as well using the Yellow Pages.