Not sure of the legal reasons, but R-134a is available at ADVANCE auto stores for about 4 months to the public. They also have retrofit kits. No FREEZE-12 though...whaaaaaa.
  BTW, if you are not from Wisconsin you may not know that buying almost ANY thing a/c related is impossible.  Because of this, shops up-charge for the "mysterious and dangerous" A/C work that the rest of the world seems to do themselves.
  Good Luck,
  Chirs C 90 MV