Then as Kermit2 said earlier, here is a circuit it uses an oscillator to pulse a transformer to give ac, but its not a pure sinewave but a square wave..use a 6-12V supply and a 3:1 transformer, it will give 18-36v ac.
http://www.electroschematics.com/wp-content/uploads/2009/10/dc-to-ac-converter.jpg
http://www.electroschematics.com/wp-content/uploads/2009/10/dc-to-ac-converter.jpg