Another complicating factor is whether you download the file or a page, print a page, or sign up for an emailed download. All three work, but some form of file will be most flexible. Usually you have to pay for that.
You probably can find books or magazines containing word jumble puzzles, but you may prefer to create the puzzles yourself. Creating the puzzles yourself has the advantage that you can customize the puzzle for different occasions, pick words suited to your child's reading level, and tailor the puzzles to your requirements (for example bu avoiding hard_to_find diagonal words in puzzles aimed at younger children). Creating puzzles yourself is possible using nothing more than a pen and paper, but you'll find it incredibly time consuming _ so a better alternative may be to use your computer and some word jumble maker software _ that way you can simply enter a list of words, and the computer will generate the puzzle automatically.