If you should run out of puzzles you can import the following Sudoku collections:

Sudoku Players' Forum

• 4593 puzzles: Beyond Nightmare
  These are the toughest of the toughest puzzles from the Sudoku Players' Forum. They are all rated >=10.1 by Sudoku Explainer and beyond anything you'll find in a newspaper, a magazine or any Sudoku app. For the most addicted players only.

OpenSudoku

• 100 puzzles: Easy
• 100 puzzles: Medium
• 100 puzzles: Hard
• 100 puzzles: Very Hard

SudoCue / SudokuVault

Education

• 160 puzzles: The Superiors Collection
• 2500 puzzles: The Learning Curve Collection

Extreme

• 400 puzzles: The Almost Daily Nightmare Collection
• 100 puzzles: 100 Snowflake Puzzles
• 3078 puzzles: The Templaters Collection
• 10000 puzzles: Top 10000 Sudokus

Civiliza's Sudoku Collection