I get mine where I get my sand, the pool and spa dealer. They normally will sell 50lb bags of Morton non-iodized table salt for $4.99. This is much cheaper than buying it from the LFS in my experience.
I use a 1 gallon pitcher to dispense it from. I just fill the pitcher up with salt and use the screw top lid to open and close it. It makes it much easier for measuring out what you need. I got the 50 gallon bag months ago and have barely put a dent in it...should last me through the year.
Cole~