How do we obtain Pokestop modules?

As of version 0.29.0, players may buy PokeStop Modules from the Store using PokeCoins. In future revisions, some Modules may be acquired from Pokestops, but that is not the case yet.

The following modules exist in the game as of the latest revision:

• Lure Module -- 100 Pokecoins
  This module allows a PokeStop to act like an Incensed player. However, it will increase the spawnrates for every player in the entire game. It also appears to be slightly more effective than Incense, at the cost of being more expensive.

As of today i have acquired a lure module from a pokestop. I have tested this theory with 5 other local players and they too have gotten ONE from them. i'm not sure if its a RNG or if at a certain point you get them. but you can get free ones at some point.