You can also save in front of Felicity, the Lucky Number lady in Goldenrod City, before midnight and wait for the date to roll over. After it does, you can talk to her and keep resetting until she draws a number that matches two digits to your Pokemon, which will give you a PP Up as a prize. You can get one a day this way.
Thanks for the responses. I was hoping not to have to cheat. The lottery and Route 45 Black Belt ideas seem to have some promise. I currently have a lv 91 Linoone with Pick Up but PP Ups are way too rare. If anyone else actually cares the Suburban Area Pokewalker course after 5000 steps has been somewhat sucessful.