They're not available in the UK as of yet, although I believe apple are working on it.
In the mean time, if you have the latest firmware, and latest version of iTunes you can create your own!
Just drag a song from iTunes to say the desktop.
Edit it in Garageband (or similar)
Convert to aac
Rename the .m4a file to .m4r
Drag back over to iTunes
et voila! It'll show up in your ringtones tab available to sync to your iPhone!