There will be a new Robert Smith mix of 'Not In Love' released Dec. 12th. You can hear a sample of it at Amazon UK. There will also be a 12" vinyl version released in January, though I'm not sure if both versions of the song will be included. The regular radio version is available now at Amazon US and iTunes US.
Update: The tracks are now available at Amazon UK and iTunes UK, but the RS remix is not available outside of the UK right now. Hope the other stores get it added soon.
Update 2: If you are buying from outside of the UK, you can get it at the War Child Music Store. If that link doesn't work, try this and then search for the track. (Thanks BH)