Sketch app users! If you are looking for iOS 11 UI kits, there’s no better place to get it than from the source. After introducing iOS11 in their annual WWDC 2017 event, Apple has made the UI kit in Sketch available for download at https://developer.apple.com/design/resources/ . You can also download the UI kit in Photoshop, and Adobe XD file types.
Aside from the iOS 11 UI kits, the link also includes UI kit downloads for WatchOS and tvOS.