NBA 2K18 The Prelude is available now on PlayStation 4 and Xbox One. You can also search for NBA 2K18 in the PlayStation and Xbox stores.
If you still have issues finding the download on PS4, check this tweet:
Here’s how you download the #NBA2K18 prelude on PS4! @Steve_OS @Ronnie2K pic.twitter.com/2VO0hmAOIq
— The Kid 👑 (@ThatKiddKuda) September 8, 2017