Austin Homebrew is usually where I've found it. It can be a little difficult sometimes as it can be sold in 500g blocks---I first "discovered" its existence while reading over on probrewer.com, where some pros even use it for their house yeast (more clean, than a liquid Kolsch would be, meaning its more versatile). In my experience, its closer to Wy1007 (Altbier from Zum Urige), but YMMV.
AHS used to pack their own 11-12g packets of the stuff. I've always liked the yeast...fast starter and strong fermenter.