According to the CDC

"Fluoride Additives for Water

What additives are used to fluoridate water?

Fluorosilicic acid is the most commonly used additive for water fluoridation, followed by two dry additives—sodium fluorosilicate and sodium fluoride. Fluorosilicic acid is derived from production of phosphate fertilizers. Phosphate rock which is comprised of calcium phosphate, calcium fluoride, apatite ore, and limestone, is mixed and heated with sulfuric acid to form a phosphoric acid-gypsum slurry, the starting point to make pelletized phosphate fertilizers. The hydrogen fluoride and silicon tetrafluoride that would otherwise be left in the gypsum slurry is deliberately recovered from the slurry by evaporators and condensed to a high purity fluorosilicic acid that can be used for water fluoridation. Both sodium fluorosilicate and sodium fluoride are created by neutralizing fluorosilicic acid with either sodium chloride (table salt) or caustic soda. See Water Fluoridation Additives for a more detailed explanation."

from CDC.gov http://www.cdc.gov/fluoridation/faqs.htm#20