If you have Verizon Wireless you have to get ringtones through their Get It Now set-up. It's a bit of a hassle having your search narrowed, however.... Namco has their own ringtone service featuring videogame music though that program. And yes, Ms. Pac Man is on the list as well as Pacmania, and other fine Namco games.
It works with other phones too so here's the link even if you don't use Verizon:
http://www.pactones.com/