I found this site http://www.zedge.net/ with free ring tones for your cell phone.
I got Kamen Rider Faiz's Henshin (transformation) as my ring tone. It sounds really cool since Faiz's powers are activated by a cell phone.
They have other Kamen Rider ring tones. Just type "Kamen Rider" in the search, and it will give you a list of the series they have.