How to power 12V LEDs from a 12V AC power supply
Not to get all techy on you, but to do what you want to do with a 12 vac power supply, you will need a bridge rectifier. Besides converting to 12vdc, you need to fill the 60 hz reverse polarity portion of the AC wave. This requires a voltage divider, then reassemble the two rectified polarity sections into one same, continuous polarity voltage source.  Keep in mind, if the AC source is sine wave AC, not switched AC, the DC result will only be apx 70% of the AC peak to peak voltage, or 17 vdc.  Think the geeks at electronics exchange may have a simple circuit  they could share with you. Realistically, since the Led lights probably have such a small amp draw, you probably could pick up a 24VDC power supply for cheap money online or maybe even your local Radio Shack, and save your self a lot of hassle.