Gav, I got mine from Revolution Racegear. Apparently they had the tinted visor "out the back somewhere" but the young bloke serving me was yawning throughout the whole process and said he couldn't find it so I paid for the helmet and walked.
It's weird, the R1 isn't mentioned on Bell's websites. The only other place I can find a visor is here:
BELL R1 - TINT VISOR [Bell R1] - Tracksformations
It says "only available in light tint". I wonder how effective that will be verses my polarised Oakleys.