There are two entities (which may or may not be connected somehow) that sell parts and kits to build watches.
Raffles DIals which sells just about everything you'd need to build a replica watch (mostly Rolex, with some Tudor and Omega parts to a lesser extent). They're very popular with builders and modders, and are known for good prices, good quality, and a very friendly user experience. Their parts also come (generally) with the proper markings and logos for whatever build you're doing. They have an
Ebay store as well, although they don't sell the branded items there because Ebay is a bit strict on copyright infringement. This is why you will hear their products referred to as "Raffles" here and "Riyi" on other forums. Same store, same guy, same items.
Raffles Time has some more specific items, but also don't sell branded items. They're very popular for hands and other items that are a bit of an upgrade from Raffles Dials products.
Both generally specialize in vintage builds...replicas of 4 and 5 digit Rolexes, especially Submariners, Explorers, OPs, Day Dates, Datjusts, and (just recently) GMTs; and their Tudor equivalents. If you want a 5513 or a 1016, it's hard to beat a DIY build from Raffles, especially for the total cost of usually under $200.