I'm not sure they sell thin kraft paper, though...
In France, I can get large thin white kraft sheets (50cm x 50cm, 30gsm) in bakery supply shops.
But it's France... there are bakeries pretty much everywhere :p
edit : here's the web site where I get them :
http://www.mon-emballage.com/papier-kra ... 1-kg-.html
The default format is 50cm x 66cm, but if you ask nicely, they'll make you a 50cm x 50cm cut.
So for 35€, you'll get something like... 1333 sheets of 50cm x 50cm

(So your stock should last a few years, even if you fold every day)