I am not a lifter, but the subject line caught my eye. We used to get chalk through gymnastics suppliers when I was a gymnast. I don't know if it is the same stuff or not or if the prices are better. We used to get it through Gibson, see below link. But you can yahoo "gymnastics chalk" and come up with several different links. Again, I don't even know if it is the same stuff that you guys use. Good luck.
I buy three or four rosin bags that baseball players use from the local sporting goods store. Easy to carry around and less mess, which is important if you workout in a commerical gym.