Chrono24
eBay
rolexforums.com
Sub007 here on RWI often has gen Sub parts for sale.
And, despite AI emerging as serious competition, there's still a good, old friend that can help you:
Uncle Google
Try search terms like: "Genuine Rolex parts for sale", or even better, be more specific if you are looking for a particular part and enter part name or part number (if you know it).
What's also great is 'Google Lens'. If you have a photo of the part in question, Lens can show you matching results under which, with some luck, sources for purchase may be among.