well welcome to the forum...glad to have you with us....well there is a great trading post here and there is lots of parts and you can use ebay too...Stefan here sells NOS parts as well...it all depends on what you are working on....God Bless John
If you own older 60's and 70's Seiko there is no place to order genuine Seiko NOS parts because they are all mostly discontinued. Typically Seiko does not sell parts directly to the public even if the parts are for a newer Seiko and available. You can find older NOS Seiko parts by hunting through eBay and posting in the sale and trade section of this forum once you are an established member here. There are a couple of good watch supply parts businesses here in the US and in the UK that have some inventory of Seiko parts but it is hit and miss.
One other avenue you can go on older Seiko's is parts watches. There is a supplier many of us use in the Philippines on eBay named thewatchcollector run by Ramon who is a pleasure to deal with and has a good stock of older Seiko parts watches that are very reasonable priced.
If you're looking for original parts that are not NOS but still usable, you can also check out speedtimerkollektion. I've mostly bought case parts from them, but they have a decent selection of parts for Seiko and Citizen. There's also Cousins UK. Your best option for NOS parts would be Stefan (seikochrono) on the forums.