Replacing 12V halogen spotlights with MR16 dimmable LEDs in my lounge
You would be better off getting a 12V DC driver/power supply, and then using a DC 12V PWM dimmer between the power supply and LEDs. 

The obvious thing to do is use a 120VAC dimmer driving the 120V->12V power supply. To begin with, AC dimmers use one of several rather awkward schemes. But even more is "lost in translation" in any 120V-12V power supply.  You are simply better off making 12VDC, and then dimming that.