American motorcycle parts can be bought both online or in an American brick and mortar store. Harley Davidson is the most well known maker of American made motorcycle parts.
There is actually a company called, "American Motorcycle Parts" that sells American made motorcycle parts. Additionally, one can purchase American made parts from Harley Davidson.
Motorcycle seats can be purchased almost anywhere that motorcycle parts and accessories are sold. A local motorcycle dealer is probably the best place to find this item.
Ocar Hedstrom inventor of the Indian Motorcycle said the E.R.Thomas was the 1st American Motorcycle sold in 1900
Yamaha used and vintage motorcycle parts can be sold with Gogo Cycles. The "parts" market is more in selling parts than buying old, and used motorcycle parts. Try a salvage yard and see if they can purchase the Yamaha motorcycle parts.
Harley Davidson is the oldest brand of American made motorcycles that is currently sold at motorcycle dealers with their first production motorcycle being sold in 1903 although the company was established in 1901. World wide it is possible that Moto Guzzi and Royal Enfield are the oldest brands being sold.
Japan
You can find motorcycle parts in a motorcycle shops that have been established 20-40 years old already.
Heads for Harley Davidson motorcycle engines are available anywhere motorcycle parts are sold. Good places to check that carry a wide variety are local dealerships and motorcycle repair shops as well as online retailers like Go Go Cycles, Phat Performance Parts and JP Cycles.
You can search on internet for online stores who can provide you jVS motorcycle parts
Thunderheader sells a variety of motorcycle parts including exhaust systems, heat shields, mounting kits, and transmission bracket clamps. One can find all of their parts for sale on their website.
Parts for a Razor motorcycle can be found at motorcycle supply stores. Check out Princess Auto, or even stores such as Amazon or eBay would have parts and accessories.
If you want to buy motorcycle parts, you can find them at http://www.motorcycle-superstore.com/227/MNGR/Motorcycle-Parts.aspx. However, if you are just looking for information about the parts of a motorcycle themselves, you can find a diagram of a motorcycle such as this one: http://wakawakaoo.com/instrument-parts-diagram-of-honda-vtr-250-for-tough-bikers/.