I used to buy them from Ian, runs this site, helped me out alot with the VFR. If you can't source one locally he's good.
http://www.tw-performance.com.au/catalo ... ucts_id=75He's moved to QLD now and i needed one just before the Rally so i went into my local bike shop and picked one up for the same price. If the BMW has a funny shaped visor it may be worth holding one up to it first to make sure they fit, they have two different sizes/shapes, one for a universal fitment and one for arai's that are a different shape or something, not sure if they are smaller or bigger, i've used the universal/shoei shape on a agv,kbc and nitro helmets
My visor didn't fog up on the inside at all when we were coming back from bathurst, or the whole weekend for that matter, just had to deal with the crap on the outside and then the tint + shit headlight in the low light conditions at the end, i would definately reccomend one for you, as you were saying, if you flick up your visor to clear it of fog, you get rain on your glasses then you have two layers of distorion, you won't need to open your visor with this.
Just make sure the visor is clean when you fit it as any marks on the inside of the visor won't be able to be cleaned once this is on and take time alignin it and making sure where you stick it on doesn't foul seals ect when the visor is closed.