http://wa0uwh.blogspot.com/2015/07/esp9266-esp12-adaptors.html
A group of three boards is $1.00 at OshPark.com (two boards are needed for each Esp-12), I have made the board available as a shared Project.
Note: I have a similar board for the Esp-13 in the works.
Eldon - WA0UWH