Word Search Puzzles is a free word search app that has tons of themes.
Each puzzle theme will change the words every time you play, so that you can go back to the same theme more then once. You will also be timed and rated so you can beat your scores by playing more. The app is free to play, but you have the option to buy more themes in game.
This app is compatible with iPhone, iPad, and iPod touch. Find it here at the App Store