I don't have a specific location but...
you may have success finding baker's ammonia in Mediterranean or Middle Eastern groceries. The brand they usually sell is called Krinos, and the product name is 'Ammonia Baking Powder.'

You can usually purchase baker's ammonia at Mediterranean or Middle Eastern groceries, but I just buy it from Amazon.

I don't know anywhere specific but I read that it might be found at mediterranean grocers, german bakeries or a pharmacy that does it's own compounding. It is also called ammonium carbonate and hartshorn.
HTH

King Arthur Flour Co carries it:

http://www.kingarthurflour.com/shop/landing.jsp?term=baker%27s+ammonia&go=DefaultSearch&stype=product

From the Cook's Thesaurus website... baker's ammonia: Originally made from the ground antlers of reindeer, this is an ancestor of modern baking powder. Northern Europeans still use it because it makes their springerle and gingerbread cookies very light and crisp. Unfortunately, it can impart an unpleasant ammonia flavor, so it's best used in cookies and pastries that are small enough to allow the ammonia odor to dissipate while baking. Look for it in German or Scandinavian markets, drug stores, baking supply stores, or a mail order catalogue. Don't confuse this with ordinary household ammonia, which is poisonous. Varieties: It comes either as lumps or powder. If it isn't powdered, crush it into a very fine powder with a mortar and pestle or a rolling pin. Substitutes (for 1 teaspoon of baker’s ammonia): 1 teaspoon baking powder (This is very similar, but might not yield as light and crisp a product.) OR 1 teaspoon baking powder plus 1 teaspoon baking soda