I've bought it from these folks:
http://www.graphic-papiers.com/categ...sp?categorie=1
Their site is in French only, but easy enough to understand and nice folks. I'm sure you could contact them with an English email.
The have it quite large, A1, A3, A4, as well as in rolls. It can get a bit pricey in the larger formats, depending on your use it seems more economical in rolls. I always need A4 which is pretty reasonable in price.
The paper shops seem to only have A4 when I look, and then it's 80gram... that weight is just light enough to make it a pain to use. 90 gram is surprisingly more stout with about the same transparency.