eBay is the best. Lots of parts, lots of good sellers. I have made 85+ eBay purchases over the past 7 years, no problems. You just have to learn how it works, and how it best works for yourself. I never bid on an item before the last two minutes of an auction, and then if i really want the item i bid to win.
J&P Cycles has just about everything you need and are said to have excellent service and parts always in stock, but higher prices.
There are some others like Jireh, Superiore, etc with the same parts as J&P but lower prices; best to enquire if they have the part in stock before ordering.
For NOS OEM parts these guys ...
[also has catalog parts]
[does not ship to USA]
For all of these it is best [necessary perhaps] to know the part numbers. For this having the factory parts catalog 99451-78B is essential. Make this your first purchase, along with 99484-78 the factory service manual.