The best place for parts is the one who has what you need at price your willing to pay!.
Their are lots of places for replacement or used parts on the web. Search for Ford 8n parts and you'll find all kinds of places. I would look at some of the MTF site sponsors first. One of the big issues for me is shipping cost. Try to buy from a company that is close by to save on shipping. I buy a lot from southern companies because the shipping is cheaper for me.
I sometime have used ford & ferguson parts from parts tractor for sale. If I can help you out at a reasonable price please let me know.
For new parts steiner and Grand valley catologs are a good place to start. Your local Napa auto parts is also a good source. You can always go the ebay route but you might not the qualilty you are looking for.