that's because the service hasn't even been launched yet. no ringtones are availble on the iTS to purchase as of yet.
if you want iTunes ringtones, download an MP3, edit to the 30 seconds you want, save it as .m4a (AAC) then change the extenstion to .mfr then drag it into the iTunes library. when you sync your iPhone, they'll show up under the ringtones tab, and will be synced to the phone.