First, there really isn't a substitute for Pickling salt. Pickling Salt is actually Kosher Salt, which is a coarse grained salt and does not contain iodine. When you're canning,
You cannot use any other salt in place of Kosher Salt/ Pickling Salt. Table salt is NOT a good substitute and should NOT be used.
Where to find Pickling Salt/ Kosher Salt:
As I mentioned, pickling salt is often only available during the summer/fall months, when canning is more prevalent. It is often found in the canning aisles and seasonal aisles of big box stores such as Target or Wal-mart. In addition many smaller hardware stores carry it year round, although generally it's pricier at these locations.
It's often cheaper to buy Kosher Salt, rather than "pickling salt", as is