There are two sources of difficult puzzles which meet the requirement of a unique solution.
For physical paper books, Will Shortz has published several at all difficulty levels including the hardest.
If you’re unfamiliar with him, he is the crossword editor for the New York Times and a llifelong self-professed gaming fanatic. He doesn’t just limit himself to crossword puzzles but Sudoku as well.
For digital puzzles, there are two IOS apps which I use daily and you can choose the difficulty level you prefer.
The first one is Sudoku Daily.
The next is Sudoku 101 and the a more advanced one. I’ll look up the developer listed for each of these and edit it back in so you’ll be sure to get the correct ones.
There are SO MANY Sudoku puzzle apps with quite similar names and I’ve probably tried most of them but these are 2–3 of the best, in my opinion.
The Sudoku Daily app is created by Phase 2 Media