I got my 2.2 lbs. from a local homebrew and wine supply (Keystone Homebrew). It cost under $7 and no shipping.
Citric Acid is used in winemaking to ajust acid levels. In cooking it's used as an antioxidant. Sprinkle some on fresh fruit and veggies to keep them from turning brown.
If you buy a 50lb. bag, I think you will have a hard time finding stuff to use it up on, unless you repack into 2.2lb bags and sell it to others wanting to do a coolant flush