You'll get opinions advocating both of these lenses, and the good news is that you can't really go wrong- they can both yield excellent results. I have used the 12mm, and found it to be so wide that I can't imagine ever shooting with it beyond the novelty of the thing. I own the 15mm lens, and like it a lot, although I find it, too, is so wide as to be of relatively limited use. I sold this lens years ago because of this, but bought another one this year, as there are indeed some situations where I've found nothing else that will get the shot. But for all 'round wide angle use, I have to say that my Voigtlander 21mm is the lens I always carry. It's still what I consider to be super-wide, and it's results are optically superb- and I find I get far more good shots with this lens over a wide range of situations than with anything wider. I have found through my own trial and error that the best 4 lens kit for me is the 21mm-35mm-50mm-90mm classic Leica set-up. Maybe that's why they were doing it for years before I was born.
As for where to buy from, I'd suggest that if a wander through the classifieds here doesn't turn one up, there are a number of the best retailers of this equipment among the sponsors of this forum- any of them would be an excellent choice. I've bought from photovillage and cameraquest and had good experiences from both.
